[plt-scheme] v299.27 parser-tools -- how do unput() or yyless()?

Subject line says it all, but if not ...

I'd like to vector to another lexer upon failure (which in my lexer means
matching ".") but I don't see how to push back "." .

'port-next-location' doesn't respect 'file-position' changes, so resetting
via file-position isn't good enough.

Hopefully I don't have to reinstantiate the input port to read up to just
before "." ?

Is an escape continuation the Scheme-y way to go here?

Toni

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