[plt-scheme] string delimiters in SLaTeX

If I say \scheme|(string->list "This is a test")|, both quotation marks 
come out as close double quotes, and the first one looks funny, because 
it slants down and to the left. If I change the first " to `` and the 
second to '', the quotes come out right, but "is a test" is coloured as 
for identifiers, not as for constants, though "This" has the right 
colour. Does anyone know how I can get properly coloured string 
constants with the right delimiters? Many thanks. --PR