[Groff] A nut to crack

[Miklos Somogyi]

Gents,

A little problem makes my hair white, because I can not reproduce it in 
any other context.

.   ps 10
.   vs .5c
.de pB          \" bulleted text
.   ne 1
.   in \\$1n
.   ps -1
\\(bu
.   ne 0
.   in +2n
.   ps
.   sp -1
..
Text above bulleted stuff
.   sp .2                                    \" in `context' it may be 
executed as .sp 1
.   pB 3
1st bulleted line
.   pB 3
2nd bulleted line                 \" in `context' and if the .sp line 
is deleted: `2nd' starts at left margin
.   pB 3
3rd bulleted line                  \" from here everything is ok
.   in 0
End of stuff
.ex

This code works fine. However, inside a bigger document either the .sp 
is not obeyed
or the second bulleted line starts at the left margin, not after the 
bullet.
If I delete a couple of ordinary text lines above this construct, 
everything is fine.
It does not matter what lines I delete.
Buffer problem? What buffer? Why not in other context?

Any idea about how to debug this problem would be much appreciated.

Miklos