[Groff] groff "overflow" and tbl empty cell problems

[andlabs]

Hello. I have two problems:

1) I am using a custom macro set for a book I'm writing, and I have a macro
to make a break. If you are at the beginning or at the end, you see a line
in the center, just like in a novel. Here is the macro:

.\" .S - Section Spacer
.\" Usage: .S
.de S
.ie \\n(.ti<=1i \{\            \" If we are at bottom, place a line and
break
.       sp 0.2
.       tl ''\(em''
.       bp \}
.el .ie \\n(.pi-1>=\\n(.t \{\  \" Else if we are at top, place a line and
space
.       sp 0.2
.       tl ''\(em''
.       sp 0.8 \}
.el .sp 1                      \" Else just space
..

With this, I have two "overflow" errors. What's going on?

PS - \n(.p is the page length (.pl), and \n(.t is the length until the next
trap (in this case, the bottom of the page trap), and troff SHOULD evaluate
expressions left-to-right, without order of operations.

2) I'm trying to make a table (not for this book) that has a single empty
cell. So I try

\&<tab>A<tab>B
\ <tab>A<tab>B
\|<tab>A<tab>B

In all cases, tbl reports that A and B are skipped ("extra data entry") or
something. I have to do

\m[white].\m[]<tab>A<tab>B

but I don't want to revert to non-portable troff code. What's going on?
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