surprising behaviour of "bgroup": sets all in greek letters

Ulf Martin wrote:

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

Looks like a bug. I'll investigate further within the next couple of 
days (except anyone else comes up with the right idea before).

Uwe Ligges

Yes, it is a bug.
I don't know whether I'm working only on the symptoms here, but the 
following patch to .../src/main/plotmath.c should fix it. I'll file it 
to R-Bugs at once.

Uwe Ligges

==========================
(diff'ed to R-1.7.0 patched (2003-05-25))
 >  diff -u ./plotmath.old ./r-patched/src/main/plotmath.c


--- ./plotmath.old     2003-05-25 17:47:23.000000000 +0200
+++ ./r-patched/src/main/plotmath.c     2003-05-25 17:49:38.000000000 +0200
@@ -2075,10 +2075,12 @@
      delim2 = DelimCode(expr, CADDDR(expr));
      bbox = RenderElement(CADDR(expr), 0);
      dist = max(bboxHeight(bbox) - axisHeight, bboxDepth(bbox) + 
axisHeight);
-    bbox = RenderDelim(delim1, dist + extra, draw);
+    if (delim1 != '.')
+        bbox = RenderDelim(delim1, dist + extra, draw);
      bbox = CombineBBoxes(bbox, RenderElement(CADDR(expr), draw));
      bbox = RenderItalicCorr(bbox, draw);
-    bbox = CombineBBoxes(bbox, RenderDelim(delim2, dist + extra, draw));
+    if (delim2 != '.')
+        bbox = CombineBBoxes(bbox,     RenderDelim(delim2, dist + 
extra, draw));
      return bbox;
  }