[bug #55154] .tr has undocumented and inconsistent space-character restrictions

[Dave]

Follow-up Comment #4, bug #55154 (project groff):

I also wonder whether this statement in groff.texi should be expanded: "The
following characters can't be translated: space (with one exception, see
below), backspace, newline, leader (and '\a'), tab (and '\t')."

It is not just ordinary space characters that cannot be translated: it is all
spaces (such as \0, \|, \^):

$ echo '.tr \|a' | groff
troff:<standard input>:1: error: expected ordinary or special character, got a
horizontal motion

One can argue that the current statement is sufficient:

1. Comment #2 points out that these other "spaces" are instead actually
horizontal motions.
2. The docs term these "escapes" rather than "characters," and .tr is
documented to work only on characters.

But there are potential rebuttals to both of these:

1. \| and \^ fall into a gray area, as they can be defined as characters in a
font file.
2. The above-quoted list of exceptions already includes two escapes.

I have no firm conclusion yet.


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