The "true branch" of a UBDD.
(qcar x) returns the true branch of the UBDD x.
For a compound UBDD, i.e., (a . b), we simply return a. But for the atomic UBDDs t and nil, which represent the constant functions t and nil, this is the identity function.
Function: qcar$inline
(defun qcar$inline (x) (declare (xargs :guard t)) (if (consp x) (car x) x))
