def stock_decode(slang):
    """Validate and translate slang string into vendor stock code.
       Our short slang must be in all lower case starting with s4,
       e.g. 's4spy' with SYMBOL in lower case.

       Using slang helps to avoid collision in our larger namespace.
    """
    if slang.isupper() or slang[:2] != 's4':
        raise ValueError('Stock slang argument is invalid.')
    else:
        try:
            symbol = slang[2:].upper()
        except Exception:
            raise ValueError('Stock slang argument is invalid.')
    return symbol
