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1 ;;; backquote.el --- Full backquote support for elisp. Reverse compatible too.
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2
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3 ;; Keywords: extensions
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4
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5 ;;; Synched up with: Not synched with FSF.
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6
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7 ;;; The bulk of the code is originally from CMU Common Lisp (original notice
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8 ;;; below).
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9 ;;;
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10 ;;; It correctly supports nested backquotes and backquoted vectors.
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11 ;;;
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12 ;;; Converted to work with elisp by Miles Bader <miles@cogsci.ed.ac.uk>
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13 ;;;
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14 ;;; Changes by Jonathan Stigelman <Stig@hackvan.com>:
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15 ;;; - Documentation added
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16 ;;; - support for old-backquote-compatibility-hook nixed because the
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17 ;;; old-backquote compatibility is now done in the reader...
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18 ;;; - nixed support for |,.| because
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19 ;;; (a) it's not in CLtl2
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20 ;;; (b) ",.foo" is the same as ". ,foo"
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21 ;;; (c) because RMS isn't interested in using this version of backquote.el
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22 ;;;
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23 ;;; wing@666.com; added ,. support back in:
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24 ;;; (a) yes, it is in CLtl2. Read closely on page 529.
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25 ;;; (b) RMS in 19.30 adds C support for ,. even if it's not really
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26 ;;; handled.
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27 ;;;
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28 ;;; **********************************************************************
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29 ;;; This code was written as part of the CMU Common Lisp project at
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30 ;;; Carnegie Mellon University, and has been placed in the public domain.
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31 ;;; If you want to use this code or any part of CMU Common Lisp, please contact
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32 ;;; Scott Fahlman or slisp-group@cs.cmu.edu.
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33 ;;;
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34 ;;; **********************************************************************
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35 ;;;
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36 ;;; BACKQUOTE: Code Spice Lispified by Lee Schumacher.
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37 ;;;
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38 ;;; The flags passed back by BQ-PROCESS-2 can be interpreted as follows:
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39 ;;;
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40 ;;; |`,|: [a] => a
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41 ;;; NIL: [a] => a ;the NIL flag is used only when a is NIL
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42 ;;; T: [a] => a ;the T flag is used when a is self-evaluating
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43 ;;; QUOTE: [a] => (QUOTE a)
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44 ;;; APPEND: [a] => (APPEND . a)
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45 ;;; NCONC: [a] => (NCONC . a)
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46 ;;; LIST: [a] => (LIST . a)
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47 ;;; LIST*: [a] => (LIST* . a)
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48 ;;;
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49 ;;; The flags are combined according to the following set of rules:
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50 ;;; ([a] means that a should be converted according to the previous table)
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51 ;;;
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52 ;;; \ car || otherwise | QUOTE or | |`,@| | |`,.|
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53 ;;;cdr \ || | T or NIL | |
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54 ;;;============================================================================
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55 ;;; |`,| ||LIST* ([a] [d]) |LIST* ([a] [d]) |APPEND (a [d]) |NCONC (a [d])
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56 ;;; NIL ||LIST ([a]) |QUOTE (a) |<hair> a |<hair> a
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57 ;;;QUOTE or T||LIST* ([a] [d]) |QUOTE (a . d) |APPEND (a [d]) |NCONC (a [d])
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58 ;;; APPEND ||LIST* ([a] [d]) |LIST* ([a] [d]) |APPEND (a . d) |NCONC (a [d])
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59 ;;; NCONC ||LIST* ([a] [d]) |LIST* ([a] [d]) |APPEND (a [d]) |NCONC (a . d)
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60 ;;; LIST ||LIST ([a] . d) |LIST ([a] . d) |APPEND (a [d]) |NCONC (a [d])
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61 ;;; LIST* ||LIST* ([a] . d) |LIST* ([a] . d) |APPEND (a [d]) |NCONC (a [d])
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62 ;;;
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63 ;;;<hair> involves starting over again pretending you had read ".,a)" instead
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64 ;;; of ",@a)"
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65 ;;;
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66
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67 ;;;
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68 ;;;
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69 ;;;
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70 ;;;
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71 ;;;
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72 ;;;
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73 ;;;
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74 ;;;
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75 ;;;
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76 ;;;
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77
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78 ;;; These are the forms it expects: |backquote| |`| |,| |,@| and |,.|.
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79 (defconst bq-backquote-marker 'backquote)
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80 (defconst bq-backtick-marker '\`) ; remnant of the old lossage
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81 (defconst bq-comma-marker '\,)
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82 (defconst bq-at-marker '\,@)
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83 (defconst bq-dot-marker '\,\.)
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84
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85 ;;; ----------------------------------------------------------------
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86
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87 (fset '\` 'backquote)
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88
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89 (defmacro backquote (template)
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90 "Expand the internal representation of a backquoted TEMPLATE into a lisp form.
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91
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92 The backquote character is like the quote character in that it prevents the
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93 template which follows it from being evaluated, except that backquote
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94 permits you to evaluate portions of the quoted template. A comma character
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95 inside TEMPLATE indicates that the following item should be evaluated. A
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96 comma character may be followed by an at-sign, which indicates that the form
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97 which follows should be evaluated and inserted and \"spliced\" into the
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98 template. Forms following ,@ must evaluate to lists.
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99
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100 Here is how to use backquotes:
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101 (setq p 'b
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102 q '(c d e))
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103 `(a ,p ,@q) -> (a b c d e)
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104 `(a . b) -> (a . b)
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105 `(a . ,p) -> (a . b)
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106
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107 The XEmacs lisp reader expands lisp backquotes as it reads them.
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108 Examples:
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109 `atom is read as (backquote atom)
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110 `(a ,b ,@(c d e)) is read as (backquote (a (\\, b) (\\,\\@ (c d e))))
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111 `(a . ,p) is read as (backquote (a \\, p))
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112
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113 \(backquote TEMPLATE) is a macro that produces code to construct TEMPLATE.
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114 Note that this is very slow in interpreted code, but fast if you compile.
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115 TEMPLATE is one or more nested lists or vectors, which are `almost quoted'.
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116 They are copied recursively, with elements preceded by comma evaluated.
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117 (backquote (a b)) == (list 'a 'b)
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118 (backquote (a [b c])) == (list 'a (vector 'b 'c))
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119
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120 However, certain special lists are not copied. They specify substitution.
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121 Lists that look like (\\, EXP) are evaluated and the result is substituted.
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122 (backquote (a (\\, (+ x 5)))) == (list 'a (+ x 5))
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123
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124 Elements of the form (\\,\\@ EXP) are evaluated and then all the elements
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125 of the result are substituted. This result must be a list; it may
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126 be `nil'.
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127
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128 Elements of the form (\\,\\. EXP) are evaluated and then all the elements
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129 of the result are concatenated to the list of preceding elements in the list.
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130 They must occur as the last element of a list (not a vector).
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131 EXP may evaluate to nil.
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132
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133 As an example, a simple macro `push' could be written:
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134 (defmacro push (v l)
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135 `(setq ,l (cons ,@(list v l))))
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136 or as
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137 (defmacro push (v l)
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138 `(setq ,l (cons ,v ,l)))
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139
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140 For backwards compatibility, old-style emacs-lisp backquotes are still read.
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141 OLD STYLE NEW STYLE
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142 (` (foo (, bar) (,@ bing))) `(foo ,bar ,@bing)
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143
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144 Because of the old-style backquote support, you cannot use a new-style
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145 backquoted form as the first element of a list. Perhaps some day this
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146 restriction will go away, but for now you should be wary of it:
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147 (`(this ,will ,@fail))
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148 ((` (but (, this) will (,@ work))))
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149 This is an extremely rare thing to need to do in lisp."
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150 (bq-process template))
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151
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152 ;;; ----------------------------------------------------------------
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153
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154 (defconst bq-comma-flag 'unquote)
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155 (defconst bq-at-flag 'unquote-splicing)
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156 (defconst bq-dot-flag 'unquote-nconc-splicing)
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157
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158 (defun bq-process (form)
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159 (let* ((flag-result (bq-process-2 form))
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160 (flag (car flag-result))
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161 (result (cdr flag-result)))
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162 (cond ((eq flag bq-at-flag)
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163 (error ",@ after ` in form: %s" form))
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164 ((eq flag bq-dot-flag)
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165 (error ",. after ` in form: %s" form))
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166 (t
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167 (bq-process-1 flag result)))))
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168
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169 ;;; ----------------------------------------------------------------
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170
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171 (defun bq-vector-contents (vec)
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172 (let ((contents nil)
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173 (n (length vec)))
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174 (while (> n 0)
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175 (setq n (1- n))
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176 (setq contents (cons (aref vec n) contents)))
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177 contents))
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178
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179 ;;; This does the expansion from table 2.
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180 (defun bq-process-2 (code)
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181 (cond ((vectorp code)
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182 (let* ((dflag-d
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183 (bq-process-2 (bq-vector-contents code))))
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184 (cons 'vector (bq-process-1 (car dflag-d) (cdr dflag-d)))))
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185 ((atom code)
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186 (cond ((null code) (cons nil nil))
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187 ((or (numberp code) (eq code t))
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188 (cons t code))
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189 (t (cons 'quote code))))
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190 ((eq (car code) bq-at-marker)
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191 (cons bq-at-flag (nth 1 code)))
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192 ((eq (car code) bq-dot-marker)
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193 (cons bq-dot-flag (nth 1 code)))
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194 ((eq (car code) bq-comma-marker)
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195 (bq-comma (nth 1 code)))
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196 ((or (eq (car code) bq-backquote-marker)
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197 (eq (car code) bq-backtick-marker)) ; old lossage
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198 (bq-process-2 (bq-process (nth 1 code))))
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199 (t (let* ((aflag-a (bq-process-2 (car code)))
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200 (aflag (car aflag-a))
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201 (a (cdr aflag-a)))
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202 (let* ((dflag-d (bq-process-2 (cdr code)))
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203 (dflag (car dflag-d))
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204 (d (cdr dflag-d)))
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205 (if (eq dflag bq-at-flag)
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206 ;; get the errors later.
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207 (error ",@ after dot in %s" code))
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208 (if (eq dflag bq-dot-flag)
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209 (error ",. after dot in %s" code))
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210 (cond
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211 ((eq aflag bq-at-flag)
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212 (if (null dflag)
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213 (bq-comma a)
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214 (cons 'append
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215 (cond ((eq dflag 'append)
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216 (cons a d ))
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217 (t (list a (bq-process-1 dflag d)))))))
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218 ((eq aflag bq-dot-flag)
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219 (if (null dflag)
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220 (bq-comma a)
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221 (cons 'nconc
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222 (cond ((eq dflag 'nconc)
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223 (cons a d))
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224 (t (list a (bq-process-1 dflag d)))))))
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225 ((null dflag)
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226 (if (memq aflag '(quote t nil))
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227 (cons 'quote (list a))
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228 (cons 'list (list (bq-process-1 aflag a)))))
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229 ((memq dflag '(quote t))
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230 (if (memq aflag '(quote t nil))
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231 (cons 'quote (cons a d ))
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232 (cons 'list* (list (bq-process-1 aflag a)
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233 (bq-process-1 dflag d)))))
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234 (t (setq a (bq-process-1 aflag a))
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235 (if (memq dflag '(list list*))
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236 (cons dflag (cons a d))
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237 (cons 'list*
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238 (list a (bq-process-1 dflag d)))))))))))
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239
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240 ;;; This handles the <hair> cases
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241 (defun bq-comma (code)
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242 (cond ((atom code)
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243 (cond ((null code)
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244 (cons nil nil))
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245 ((or (numberp code) (eq code 't))
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246 (cons t code))
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247 (t (cons bq-comma-flag code))))
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248 ((eq (car code) 'quote)
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249 (cons (car code) (car (cdr code))))
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250 ((memq (car code) '(append list list* nconc))
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251 (cons (car code) (cdr code)))
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252 ((eq (car code) 'cons)
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253 (cons 'list* (cdr code)))
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254 (t (cons bq-comma-flag code))))
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255
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256 ;;; This handles table 1.
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257 (defun bq-process-1 (flag thing)
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258 (cond ((or (eq flag bq-comma-flag)
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259 (memq flag '(t nil)))
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260 thing)
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261 ((eq flag 'quote)
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262 (list 'quote thing))
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263 ((eq flag 'vector)
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264 (list 'apply '(function vector) thing))
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265 (t (cons (cdr
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266 (assq flag
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267 '((cons . cons)
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268 (list* . bq-list*)
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269 (list . list)
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270 (append . append)
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271 (nconc . nconc))))
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272 thing))))
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273
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274 ;;; ----------------------------------------------------------------
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275
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276 (defmacro bq-list* (&rest args)
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277 "Returns a list of its arguments with last cons a dotted pair."
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278 (setq args (reverse args))
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279 (let ((result (car args)))
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280 (setq args (cdr args))
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281 (while args
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282 (setq result (list 'cons (car args) result))
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283 (setq args (cdr args)))
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284 result))
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285
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286 (provide 'backquote)
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287
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