[R] surprising behaviour of "bgroup": sets all in greek letters

[Ulf Martin]

Dear R user community

I wanted to use "bgroup" for plotting a math formula with
a big "{" on the left, and nothing on the right.
i used

  text( 10, 10, pos=4, cex=1.8, expression(F(x) == bgroup("{", x, "")), ...)

on a 40 x 20 plot.
surprisingly,
bgroup sets "Phi(xi) = { xi" 
i.e. replaces alphabetic characters with greek letters in the entire formula.

I tried out other ending delimiters instead of "":
With " ", ":", ";", ",", R complains about an "invalid group delimiter",
With "." R produces greek letters again.

Is this a bug, or am I missing something?
How could I possibly accomplish my original task?
The idea is to have something like

          0 if x<0
 F(x) = {
          y otherwise

in the end.

Thanks in advance!
(and for the great tool anyway!)

(I am not currently subscribed to the r-help list,
so I would be gratful if anybody could answer me directly too, thanks!)

Bye
-- 
ulf martin
ulf.martin at gmx.net