Sadly Doug, I am nowhere near as economical with words as you!
On Mon, 22 May 2023, Douglas McIlroy wrote:
Basically, [if] a tab occurs within braces, it will be rejected.
Running eqn standalone, I got identical outputs from these two inputs
.EQ
a<tab>b
.EN
.EQ
{a<tab>b}
.EN
The tab is passed to groff as \t\, man 7 groff says \t is
"uninterpreted", yet the tab skips to a tab stop set by .ta.
This leaves me totally confused, because groff apparently ignores \t
when it doesn't come from eqn.
If, as i believe, eqn doesn't know about .ta and groff doesn't know
about eqn and normally ignores \t, what causes the skip to happen?
Sorry Doug, I do not know. A gap in my knowledge.
That said, I think it is a bit more confusing than this. Taking the EQN
expression which is the solution of a quadratic equation
a x sup 2 + b x + c = 0
from Ted Harding's document (which also uses braces), I put a tab OUTSIDE
the EQN solution expression in braces and then subsequently, a tab INSIDE
the braces in that same expression. While standalone eqn is very happy to
process the first, it rejects the second. I am using 1.22.4. The text is
.\" a tab outside the expression in braces seems OK
.EQ
x ~ = ~~
<tab> { - b +- sqrt { b sup 2 - 4 a c } }
over {2 a}
.EN
.sp
.\" a tab inside the brace causes 'eqn' to complain with
.\"
.\" eqn:zz:16: tabs allowed only at outermost level
.\"
.EQ
x ~ = ~~
{ <tab> - b +- sqrt { b sup 2 - 4 a c } }
over {2 a}
.EN
So, a <tab> inside the expression wrapped in braces is an error! That
said, in your far simpler example Doug, it is not. I am very lost.
For this error, I believe that 'eqn' is supposed to respect the existing
'.ta' requests which means it cannot allow them inside an expression which
it wants to set.
So, I would like to put in a user guide words like
Avoid (like the plague) a tab anywhere between .EQ ... .EN or $ ... $
for the case where those '$' are in-line EQN delimiters.
I find it is not worth exposing eqn to a <tab> anywhere.
I think my idea is good advice but does it belong in a user guide!
Thanks - Damian
