How to approach refined grammar rules

[alexanderchen750]

I'm trying to define a cfg and add some refinement rule on top of that. For example using the cfg expression defined in the github readme:

import guidance
from guidance import one_or_more, select, zero_or_more
# stateless=True indicates this function does not depend on LLM generations
@guidance(stateless=True)
def number(lm):
    n = one_or_more(select(['0', '1', '2', '3', '4', '5', '6', '7', '8', '9']))
    # Allow for negative or positive numbers
    return lm + select(['-' + n, n])

@guidance(stateless=True)
def operator(lm):
    return lm + select(['+' , '*', '**', '/', '-'], name="op")

@guidance(stateless=True)
def expression(lm):
    # Either
    # 1. A number (terminal)
    # 2. two expressions with an operator and optional whitespace
    # 3. An expression with parentheses around it
    return lm + select([
        number(),
        expression() + zero_or_more(' ') +  operator() + zero_or_more(' ') +  expression(),
        '(' + expression() + ')'
    ])

I'd like to add a rule to stop a zero from being chosen if the previous operator was a '/'. I though about accessing the state of the lm like lm["op"], but stateless functions can't access state. If I set it to stateful, I run into error that I can't select between stateful functions. I thought about using a table to store state, like table["op"] = select(['+' , '*', '**', '/', '-'], name="op"), but it stores a intermediary representation like {{G|137482486511456|G}}.

I'm still new to guidance, so I would appreciate some pointers in the right direction.