Npn4s-length
Get the length of the npn4arr array.
In the execution this just gets the array length. The
logical definition is just a wrapper for len:
Function: npn4s$a-length
(defun npn4s$a-length (npn4arr$a)
(declare (xargs :guard (npn4arr$ap npn4arr$a)))
(len npn4arr$a))
