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