def _find(string, sub_string, start_index):
result = string.find(sub_string, start_index)<EOL>if result == -<NUM_LIT:1>:<EOL><INDENT>raise TokenError("<STR_LIT>".format(sub_string))<EOL><DEDENT>return result<EOL>
Return index of sub_string in string.

    Raise TokenError if sub_string is not found.