length of a list
Major Section:  ACL2-BUILT-INS

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.)

To see the ACL2 definition of this function, see pf.