[Groff] parsing a corner specification

[Joel E. Denny]

I am trying to understand GNU pic's parser as it relates to an extension I 
am working on for Bison.  However, I've never used pic before, so some of 
my questions may be naive.

I checked out Groff from CVS and built it.  The most recent ChangeLog 
entry at the time was 2007-07-15.

I then ran:

  % cat > test.pic
  .PS
  A: circle "A"
  B: A left
  circle "B" at B
  .PE
  % src/preproc/pic/pic test.pic > test.groff
  src/preproc/pic/pic:test.pic:4: syntax error before `left'
  src/preproc/pic/pic:test.pic:4: giving up on this picture

However:

  % cat > test.pic
  .PS
  A: circle "A"
  B: A upper left
  circle "B" at B
  .PE
  % src/preproc/pic/pic test.pic > test.groff

There is no error, and the circle labeled B is positioned at the upper 
left corner of the circle labeled A.

The only difference in the input is the word "upper".  Is this apparent 
discrepancy in behavior intended?  If not, should both or neither of these 
inputs be a syntax error?

I've searched the web and Open Group for some roff or pic standards 
document.  So far, I haven't found one.  Does one exist?  `man roff' says 
Groff is the de facto standard.  Is that still the closest we have to a 
formal standard?

Thanks.