Takes an action in the current grammar state, returning a new grammar state with whatever
        updates are necessary.  The production rule is assumed to be formatted as "LHS -> RHS".

        This will update the non-terminal stack and the context-dependent actions.  Updating the
        non-terminal stack involves popping the non-terminal that was expanded off of the stack,
        then pushing on any non-terminals in the production rule back on the stack.  We push the
        non-terminals on in `reverse` order, so that the first non-terminal in the production rule
        gets popped off the stack first.

        For example, if our current ``nonterminal_stack`` is ``["r", "<e,r>", "d"]``, and
        ``action`` is ``d -> [<e,d>, e]``, the resulting stack will be ``["r", "<e,r>", "e",
        "<e,d>"]``.

def take_action(self, production_rule: str) -> 'LambdaGrammarStatelet':
        """
        Takes an action in the current grammar state, returning a new grammar state with whatever
        updates are necessary.  The production rule is assumed to be formatted as "LHS -> RHS".

        This will update the non-terminal stack and the context-dependent actions.  Updating the
        non-terminal stack involves popping the non-terminal that was expanded off of the stack,
        then pushing on any non-terminals in the production rule back on the stack.  We push the
        non-terminals on in `reverse` order, so that the first non-terminal in the production rule
        gets popped off the stack first.

        For example, if our current ``nonterminal_stack`` is ``["r", "<e,r>", "d"]``, and
        ``action`` is ``d -> [<e,d>, e]``, the resulting stack will be ``["r", "<e,r>", "e",
        "<e,d>"]``.
        """
        left_side, right_side = production_rule.split(' -> ')
        assert self._nonterminal_stack[-1] == left_side, (f"Tried to expand {self._nonterminal_stack[-1]}"
                                                          f"but got rule {left_side} -> {right_side}")
        assert all(self._lambda_stacks[key][-1] == left_side for key in self._lambda_stacks)

        new_stack = self._nonterminal_stack[:-1]
        new_lambda_stacks = {key: self._lambda_stacks[key][:-1] for key in self._lambda_stacks}

        productions = self._get_productions_from_string(right_side)
        # Looking for lambda productions, but not for cells or columns with the word "lambda" in
        # them.
        if 'lambda' in productions[0] and 'fb:' not in productions[0]:
            production = productions[0]
            if production[0] == "'" and production[-1] == "'":
                # The production rule with a lambda is typically "<t,d> -> ['lambda x', d]".  We
                # need to strip the quotes.
                production = production[1:-1]
            lambda_variable = production.split(' ')[1]
            # The left side must be formatted as "<t,d>", where "t" is the type of the lambda
            # variable, and "d" is the return type of the lambda function.  We need to pull out the
            # "t" here.  TODO(mattg): this is pretty limiting, but I'm not sure how general we
            # should make this.
            if len(left_side) != 5:
                raise NotImplementedError("Can't handle this type yet:", left_side)
            lambda_type = left_side[1]
            new_lambda_stacks[(lambda_type, lambda_variable)] = []

        for production in reversed(productions):
            if self._is_nonterminal(production):
                new_stack.append(production)
                for lambda_stack in new_lambda_stacks.values():
                    lambda_stack.append(production)

        # If any of the lambda stacks have now become empty, we remove them from our dictionary.
        new_lambda_stacks = {key: new_lambda_stacks[key]
                             for key in new_lambda_stacks if new_lambda_stacks[key]}

        return LambdaGrammarStatelet(nonterminal_stack=new_stack,
                                     lambda_stacks=new_lambda_stacks,
                                     valid_actions=self._valid_actions,
                                     context_actions=self._context_actions,
                                     is_nonterminal=self._is_nonterminal)