I am trying out a prototype for a language using the textx module for python that can interpret grammars.
I am creating a model like this:

from textx import metamodel_from_str

funl_grammar = """
    Model: statement *= Statement;

    Statement: FunctionDefinition | Function;

    FunctionDefinition: name=ID '=' function=Function;
    Function: name=ID '(' params*=Param (',' params*=Param)* ')';

    Param: Function | INT | ID | STRING;
    Comment: /#.*$/;
    ID: /[a-zA-Z_][a-zA-Z0-9_]*/;
    INT: /[0-9]+/;
    STRING: /".*"/;
"""

mm = metamodel_from_str(funl_grammar)

Now my question is, how can I typecheck for it using mypy?

If I create a function like this where I want to be sure that the parameter is part of my grammar and textx considers it a Param:

def my_func(param: mm["Param"]):
    ...

I get the mypy error error: Name "Param" is not defined [name-defined].

Do you know how I could fix that? Googling and ChatGPT has not helped so far.