ANSI Common Lisp 3 Evaluation and Compilation 3.4 Lambda Lists
3.4.11 Syntactic Interaction of Documentation Strings and Declarations
In a number of situations, a documentation string can appear amidst a
series of declare expressions prior to a series of forms.
In that case, if a string S appears where a documentation string is
permissible and is not followed by
either a declare expression
or a form
then S is taken to be a form;
otherwise, S is taken as a documentation string.
The consequences are unspecified if more than one such documentation string