Builds a boolean vector for addition.
Builds a new boolean vector that represents the addition 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
![\[ c_i = (l_i\ \mbox{and}\ r_i)\ \mbox{or}\ (c_{i-1}\ \mbox{and} \ (l_i\ \mbox{or}\ r_i)) \]](form_32.png)
It is important for efficency that the BDD variables used in l and r are interleaved.
- Returns:
- The result of the addition (which is already reference counted).
