#+ provides a read-time conditionalization facility; the syntax is #+test expression. If the feature expression test succeeds, then this textual notation represents an object whose printed representation is expression. If the feature expression test fails, then this textual notation is treated as whitespace; that is, it is as if the ``#+ test expression'' did not appear and only a space appeared in its place.
For a detailed description of success and failure in feature expressions, see Section 220.127.116.11 (Feature Expressions).
#+ operates by first reading the feature expression and then skipping over the form if the feature expression fails. While reading the test, the current package is the KEYWORD package. Skipping over the form is accomplished by binding *read-suppress* to true and then calling read.
For examples, see Section 18.104.22.168.1 (Examples of Feature Expressions).