[pstricks] computing coordinates

[Nitecki, Zbigniew H.]

I know I have had this answered before, but can't remember how it works.

I want to draw a family of horizontal lines (with arrowheads) filling out a parallelogram, bounded by
the lines y=-x and y=-(x+2) from y=0 to y=-2.  I am trying to do it via \multido, so for t=0 to t=2 in increments of d,
I want to repeat \psline{->}(t,-t)(t+2,-t).  I know that
\multido{\rx=0.0+0.2}{11}{\psline(\rx,-\rx)(2+\rx,-\rx)}
doesn't work, because postscript gets confused by "2+\rx".
So my question is, how do I specify coordinates of a point to be computed from a parameter?
I seem to recall there are two systems, one involving an exclamation point and the other a postscript reverse Polish format, but haven't on a quick search found the appropriate information in LGC2 or Herbert's book.

Can someone (again) explain how this is done?

Zbigniew Nitecki
Department of Mathematics
Tufts University
Medford, MA 02155

telephones:
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http://www.tufts.edu/~znitecki/