cond-expand

(cond-expand (requirement body...)... [(else body...)])

Takes the first clause whose requirement holds, else the else clause, and stands for its body as a begin would: at the top level and in a body its definitions are definitions there, around the leading imports its imports are the program's, and in an expression it is the value of the last form. As a declaration of a define-library its body is more declarations. A requirement is:

• a feature identifier, one of those features returns;
• (library name), which holds when (import name) would find the library: a standard library this front end has, a define-library before it, or a library file;
• (and requirement...), (or requirement...), (not requirement).

The clause is chosen when the program is read, so a compiled program carries only the body it took. A cond-expand no clause of which holds, with no else, is an error; R7RS leaves it unspecified.

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