[racket] SGL Bindings

Hello all,

I'm attempting to do some 3d targeting by screen position, and it would 
be convenient to use unproject, however I can't figure out the 
parameters.  The OpenGL specs require various matrices to be passed in, 
and Racket wants them of type gl-double-vector, which seems to be a type 
synonym for cvector, but then when I get the matrices of that type with 
glGetFloatv and pass it to unproject it complains it got a cvector 
instead of a double-vector...

Does anybody know how to use unproject?