[pstricks] Correct use of \coor

[Andrew Parsloe]

Hullo,

A first posting to this list. Can someone give me an example in which 
the PSTricks command \coor is used correctly? The manual says "Convert 
one or more PSTricks coordinates to a pair of numbers (using pt units), 
and insert them in the Postscript code." This leads me to suppose that 
(by way of illustration)

\pscustom{\SpecialCoor\psline(!\coor(3,4))}

should draw a line from the origin to (3,4), but it doesn't. I get an 
error message "Argument of \coor has an extra }. Paragraph ended before 
\coor was complete. Paragraph ended before \special at coor was complete." 
What am I doing wrong?

Andrew