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  24 System Construction   24.1 System Construction Concepts   24.1.2 Features   24.1.2.1 Feature Expressions

24.1.2.1.1 Examples of Feature Expressions

For example, suppose that in implementation A, the features spice and perq are present, but the feature lispm is not present; in implementation B, the feature lispm is present, but the features spice and perq are not present; and in implementation C, none of the features spice, lispm, or perq are present. The next figure shows some sample expressions, and how they would be read2 in these implementations.

Features examples
(cons #+spice "Spice" #-spice "Lispm" x)
   in implementation A ... (CONS "Spice" X)
   in implementation B ... (CONS "Lispm" X)
   in implementation C ... (CONS "Lispm" X)
(cons #+spice "Spice" #+LispM "Lispm" x)
   in implementation A ... (CONS "Spice" X)
   in implementation B ... (CONS "Lispm" X)
   in implementation C ... (CONS X)
(setq a '(1 2 #+perq 43 #+(not perq) 27))
   in implementation A ... (SETQ A '(1 2 43))
   in implementation B ... (SETQ A '(1 2 27))
   in implementation C ... (SETQ A '(1 2 27))
(let ((a 3) #+(or spice lispm) (b 3)) (foo a))
   in implementation A ... (LET ((A 3) (B 3)) (FOO A))
   in implementation B ... (LET ((A 3) (B 3)) (FOO A))
   in implementation C ... (LET ((A 3)) (FOO A))
(cons #+Lispm "#+Spice" #+Spice "foo" #-(or Lispm Spice) 7 x)
   in implementation A ... (CONS "foo" X)
   in implementation B ... (CONS "#+Spice" X)
   in implementation C ... (CONS 7 X)


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