def stacks_2_eqns(self,stacks):
        """returns equation strings from stacks"""
        if stacks:
            return list(map(lambda p: self.stack_2_eqn(p), stacks))
        else:
            return []
def stacks_2_eqns ( self , stacks ) : if stacks : return list ( map ( lambda p : self . stack_2_eqn ( p ) , stacks ) ) else : return [ ]
returns equation strings from stacks