proofs.py

    def is_valid(self):
        """ Return whether lines are a valid proof of statement from rules """
        # Task 4.6
        for i,line in enumerate(self.lines):
            # If it doesn't have a rule, we assume it  is an assumption
            if line.rule is None:
                if line.conclusion not in self.statement.assumptions:
                    return False
                else:
                    continue
            rule = self.rules[line.rule]
            if not self.instance_for_line(i).is_instance_of(rule):
                return False
            if False in [i > x for x in line.justification]:
                return False
        return self.instance_for_line(len(self.lines) - 1).conclusion.infix()\
               == self.statement.conclusion.infix()