Subparsers are added like:

subparsers.add_parser('checkout', aliases=['co'])

But I want to use a parser BOTH as a subparser and as a full-fledged parser.

It is because my program should understand both of the following command line options:

boiler chain -t http://www.w3.org/1999/xhtml -W inverseofsum

and

boiler pipe 'chain -t http://www.w3.org/1999/xhtml -W inverseofsum + transformation http://example.com/ns1'.

I split it (at +) into several lists of arguments as explained in https://stackoverflow.com/a/53750697/856090

So I need `chain` both as a subparser and as a standalone parser of `-t http://www.w3.org/1999/xhtml -W inverseofsum`.

So, feature which I want:

subparsers.add_parser('checkout', aliases=['co'], parser=...)

where ... is a reference to a parser object.