def _assign_match(LL: list[Leaf]) -> Optional[int]:
        """
        Returns:
            string_idx such that @LL[string_idx] is equal to our target (i.e.
            matched) string, if this line matches the assignment statement
            requirements listed in the 'Requirements' section of this classes'
            docstring.
                OR
            None, otherwise.
        """
        # If this line is a part of an expression statement or is a function
        # argument AND the first leaf contains a variable name...
        if (
            parent_type(LL[0]) in [syms.expr_stmt, syms.argument, syms.power]
            and LL[0].type == token.NAME
        ):
            is_valid_index = is_valid_index_factory(LL)

            for i, leaf in enumerate(LL):
                # We MUST find either an '=' or '+=' symbol...
                if leaf.type in [token.EQUAL, token.PLUSEQUAL]:
                    idx = i + 2 if is_empty_par(LL[i + 1]) else i + 1

                    # That symbol MUST be followed by a string...
                    if is_valid_index(idx) and LL[idx].type == token.STRING:
                        string_idx = idx

                        # Skip the string trailer, if one exists.
                        string_parser = StringParser()
                        idx = string_parser.parse(LL, string_idx)

                        # The next leaf MAY be a comma iff this line is a part
                        # of a function argument...
                        if (
                            parent_type(LL[0]) == syms.argument
                            and is_valid_index(idx)
                            and LL[idx].type == token.COMMA
                        ):
                            idx += 1

                        # But no more leaves are allowed...
                        if not is_valid_index(idx):
                            return string_idx

        return None

        Returns:
            string_idx such that @LL[string_idx] is equal to our target (i.e.
            matched) string, if this line matches the assignment statement
            requirements listed in the 'Requirements' section of this classes'
            docstring.
                OR
            None, otherwise.
        
src/black/trans.py