An alphanumeric identifier suitable for ordinary values. Not an identifier. The comma is not permitted in identifiers of any kind. The characters of symbolic and alphanumeric identifiers may not be combined in a single identifier. There is no fourth component.
Function implode applies only to lists of characters.
Here, the argument is a tuple of characters. Function tl cannot be applied to the empty list, whose tail is not defined. Function concat applies to lists of strings, not lists of characters.