Builds a boolean vector for subtraction.
Builds a new boolean vector that represents the subtraction of two other vectors. Each element 
in the result will represent the function
![\[ x_i = l_i\ \mbox{xor}\ r_i\ \mbox{xor}\ c_{i-1} \]](form_30.png)
where the carry in 
is
![\[ c_i = (l_i\ \mbox{and}\ r_i\ \mbox{and}\ c_{i-1})\ \mbox{or}\ (\mbox{not}\ l_i\ \mbox{and}\ (r_i\ \mbox{or}\ c_{i-1})) \]](form_33.png)
It is important for efficency that the BDD variables used in l and r are interleaved.
- Returns:
- The result of the subtraction (which is already reference counted).
