[pstricks] Version of \pscircle for use on Postscript side

[Michael Sharpe]

I was looking for a way to use a macro like \pscircle, but with the  
ability to use \SpecialCoor to specify all parameters. That's possible  
now with all except the radius. (I realize that the pst-eucl package  
contains a number of special cases allowing construction of circles  
from nodes, but unless I'm being obtuse, there is not a macro that  
works in the generality I wanted.) To cut a long story short, it turns  
out that the answer was already there in pstricks---\psellipse. The  
fact that you specify the two radii as a pair (a,b) means that the  
parser allows Postscript code for a,b, or a \pnode. For example:

\SpecialCoor
\pnode(2,2){r}%radius=2
\pnode(1,1){C}%center
\pnode(2.1,1.5){D}\psdot(D)
\psellipse(C)(r)
\psLCNode(C){-1}(D){1}{R}% R=CtoD
\psellipse(C)(! \psGetNodeCenter{R} R.x R.y Pyth dup) %circle, center  
C, through D