LEN.html -- ACL2 Version 6.3

### LEN

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.