Return a list of all the literals contained in this expression.
        Include recursively subexpressions symbols.
        This includes duplicates.

def get_literals(self):
        """
        Return a list of all the literals contained in this expression.
        Include recursively subexpressions symbols.
        This includes duplicates.
        """
        if self.isliteral:
            return [self]
        if not self.args:
            return []
        return list(itertools.chain.from_iterable(arg.get_literals() for arg in self.args))