[plt-scheme] How to "unput" characters to a port during lexing?

The parser for CSS2.1 is now almost finished. The grammar
used is the flex/yacc one from

     <http://www.w3.org/TR/CSS21/grammar.html>.

There is just one rule left, that I can't figure out
how to port.

It is the line

   {s}+\/\*[^*]*\*+([^/*][^*]*\*+)*\/	{unput(' ');

which puts a space back in the input stream.

How do I "unput" a space in the parser-tools lexer?

-- 
Jens Axel Søgaard