the toffoli gate is universal; this means that for any boolean function "f"("x", "x", ..., "x"), there is a circuit consisting of toffoli gates that takes "x", "x", ..., "x" and some extra bits set to 0 or 1 to outputs "x", "x", ..., "x", "f"("x", "x", ..., "x"), and some extra bits (called garbage). essentially, this means that one can use toffoli gates to build systems that will perform any desired boolean function computation in a reversible manner.