groff now ignores .so directive

[lukekendall]

groff 1.17.2
groff -t -mm -L11.0i zzz > x.ps
Unknown macro `.St' on line 11 of zzz

zzz:

..so prob.i
..S 10 13
..de Xa
..ad
...
..Xa
..fi
..HU "Prologue"
..P
Blather
..St
More blather

prob.i:

'\" -- Small section separation (minor scene changes):
..de St
..ne 4
..sp 2
...
'\" -- Medium section separation (minor scene changes):
..de Sm
..ne 4
..sp 1
'\".ce
'\"*~*~*
'\" *
'\".sp 1
...
'\" -- Stars for separating sections with greater scene changes:
..de ST
..ne 4
..sp 1
..ce
'\" ***~*~*~*~~*~~*~*~*~***
*~*~*
..sp 1
...

If you literally include prob.i via inserting it into zzz with a text
editor, the warning vanishes.

luke