len, which is tail-recursive and fixnum-aware, that
differs from its simple ACL2 definition.)
Major Section: PROGRAMMING
Len returns the length of a list.
A Common Lisp function that is appropriate for both strings and
proper lists is length; see length. The guard for len is t.
(Low-level implementation note. ACL2 provides a highly-optimized
implementation of len, which is tail-recursive and fixnum-aware, that
differs from its simple ACL2 definition.)