[pstricks] variables and pstricks.

[Eduardo]

Hi, 

Is it possible to define variables using pstricks? 

The following commands work fine:
\def\R{2}
\pscircle(0,0){\R} 

but I'd like to have, for example: 
\SpecialCoor
!/R 2 sqrt def 
\pscircle(0,0){R} 

Thanks in advance, 

Eduardo.