converting page coordinate to user coordinate

[bngsk]

Hi,

Can any one help me with a postscript procedure that converts page coordinates (x y) to user coordinates (page-to-user). This should return on the stack the user coordinates x' y'.

User ----C
| |
| |
| V
| Physical
| ^
| |
| |
Page----D

The following converts User coordinates (X Y))to Page coordinates:
X Y TRANSFORM MATRIX DEFAULTMATRIX ITRANSFORM

How to achieve the page --> user coordinate conversion?

Thanks,