'Retract assumptions from the assumption list.
:param debug: If True, give warning when ``retracted`` is not present on
assumptions list.
:type debug: bool
:param retracted: assumptions to be retracted
:type retracted: list(sem.Expression)'

def retract_assumptions(self, retracted, debug=False):

    retracted = set(retracted)
    result_list = list(filter((lambda a: (a not in retracted)), self._assumptions))
    if (debug and (result_list == self._assumptions)):
        print(Warning('Assumptions    list    has    not    been    changed:'))
        self.print_assumptions()
    self._assumptions = result_list
    self._result = None
