ANSI Common Lisp   2 Syntax   2.4 Standard Macro Characters   2.4.8 Sharpsign

2.4.8.18 Sharpsign Minus

#- is like #+ except that it skips the expression if the test succeeds; that is,

#-test expression ==#+(not test) expression

For examples, see Section 24.1.2.1.1 Examples of Feature Expressions.