'See docstring of SeqBase._mul'

def _mul(self, other):

    if isinstance(other, SeqFormula):
        (form1, v1) = (self.formula, self.variables[0])
        (form2, v2) = (other.formula, other.variables[0])
        formula = (form1 * form2.subs(v2, v1))
        (start, stop) = self._intersect_interval(other)
        return SeqFormula(formula, (v1, start, stop))
