Multiply two matrices and replace current one.

def concat(self, one, two):
        """Multiply two matrices and replace current one."""
        if not len(one) == len(two) == 6:
            raise ValueError("bad sequ. length")
        self.a, self.b, self.c, self.d, self.e, self.f = TOOLS._concat_matrix(one, two)
        return self