[mom] R_MARGIN ignored

[Frederic Chartier]

I'm having trouble controlling the margins with -mom, especially
the right one. Consider this input :

.PAPER A4
.PRINTSTYLE TYPESET
.L_MARGIN 1c
.R_MARGIN 1c
.DOCHEADER OFF
.FOOTERS
.FOOTER_ON_FIRST_PAGE
.START
.PP
Some text. Some text. Some text. Some text. Some text. Some text.
Some text. Some text. Some text. Some text. Some text. Some text.
Some text. Some text. Some text. Some text. Some text. Some text.
.TS H
box expand;
l r .
.TH
left    right
.TE
.PP
Some more text. Some more text. Some more text. Some more text.
Some more text. Some more text. Some more text. Some more text.
Some more text. Some more text. Some more text. Some more text.

L_MARGIN is honoured but R_MARGIN is ignored. The paragraphs,
the table and the rule at the foot of the page all stop much
further than 1 cm away from the right edge of the paper. In
fact, it looks like the default right margin (1 inch) is still
in force. I must be doing something wrong, but what ? Thanks in
advance.

(This is with om.tmac from mom 2.5_c.)