🚀 go-pugleaf

I didn't get a satisfactory answer last time I posed this question so
now I will turn it into a challenge to see if I get a better response. :-)
Can you implement TeX's equivalent of an \halign in troff?  What I want
to do is create hanging tags where the longest hanging tag is the width
of the paragraph indent.  For example, I would like to get the output

	/etc/motd	     The  message  of the day.  Continuation lines
			     such as these continue here.
	/usr/lib/tmac/tmac.s The  -ms  macros.   Notice  that  the longest 
			     hanging tag is used as the indent for all the
			     paragraphs
	
Also, how does the net world debug troff macros?  The .tm request helps
somewhat but it is always executed as soon as it read and you cannot delay
its output to print out when the macro is executed, not when it is defined.
		Scott Simpson

Here is my non-working (but close) solution to the above problem.  I don't
know why it doesn't work.  I'm using Sun OS 3.5 [nt]roff.

.\" %W% %G%
.\" General purpose macros.	Scott Simpson.
.\" Macros provided:
.\"	.FB file_name		Begin a File Section line in the -man
.\"				macros.  For example, in the -man section
.\"				where you say .SH FILES, you could say
.\"					.FB /etc/motd
.\"					The message of the day.
.\"					.FE
.\"					.FS
.\"				to format the file line.  .TP inserts a 
.\"				blank line so we don't use it.  Each .FB
.\"				must end with a .FE and a final .FS must end
.\"				the last .FB-.FE pair.  The widest tag is
.\"				computed and used for all the .FS-.FE pairs.
.\"	.FE			Terminates an .FB command.
.\"	.FS			Flushes a file section.
.\"
.\" This file uses the following:
.\"	Number registers:
.\"		mw	maximum width of a hanging tag
.\"	Macros, diversions and strings:
.\"		FD	diversion for .FB, .FE, .FS
.\"
.nr mw 0	\"indent size of hanging tag
.de FB		\"File Begin section
.if \w'\\$1'u+1m>\\n(mw .nr mw \w'\\$1'u+1m
.nf		\"get output in no-fill mode
.da FD		\"start diversion
.ft R		\"put environment in sane state
.ps 10
.vs 12p
.br		\"start a new paragraph
.ti -\\n(mwu	\"dedent
\\$1\\t\c
..
.de FE		\"File section End
.di		\"end diversion
..
.de FS		\"Flush Section
.in +\\n(mwu	\"set indentation to size of largest tag
.ta 0		\"set tab to indent size
.fi		\"set fill mode
.FD		\"output diversion
.\" Adding a comment to the end of the next line causes an infinite loop!
.\" Remove the macro request.
.rm FD
.in -\\n(mwu	\"dedent indent
.nr mw 0	\"reset hanging tag size
..
	Scott Simpson
	TRW Space and Defense Sector
	oberon!trwarcadia!simpson  	(UUCP)
	trwarcadia!simpson@usc.edu	(Internet)