Rendering of math symbols in ASCII mode

["Anthony J. Bentley" <anthony@anjbe.name>]

Hi,

This beauty shows up in erf(3):

.Bd -filled -offset indent
.if n \{\
erf(x) = 2/sqrt(pi)*\|integral from 0 to x of exp(\-t*t) dt. \}
.if t \{\
erf\|(x) :=
(2/\(sr pi)\|\(is\d\s8\z0\s10\u\u\s8x\s10\d\|exp(\-t\u\s82\s10\d)\|dt. \}
.Ed

My first instinct was to replace it with eqn(7), a pleasant way to write
non-trivial equations:

.EQ
roman erf ( x ) = 2 over {sqrt pi} int sub 0 sup x e sup {- t sup 2} dt
.EN

This looks great in groff -Tpdf and mandoc -Thtml, and is serviceable
enough in mandoc -Tutf8. Unfortunately, in ASCII it looks like this:

    erf ( x ) = 2/(sqrt(n)) I_0^x e^(- t^2) dt

As per mandoc_char(7), the integral symbol is replaced with I and pi
is replaced with n. Awful!

While I'm inclined to think that attempting to render equations in pure
ASCII is somewhat futile, the human-readable nature of eqn(7) suggests
the possibility of simply displaying the symbol name for certain symbols
like Greek letters and integrals.

    erf ( x ) = 2/(sqrt(pi)) int_0^x e^(- t^2) dt

I think it compares favorably with the current rendering:

    erf(x) = 2/sqrt(pi)*integral from 0 to x of exp(-t*t) dt

This would make it practical to use eqn(7) in certain libm manuals,
which I think could be a nice improvement. Viewing the math-heavy libGL
manuals on man.openbsd.org is a real pleasure.

Thoughts?

-- 
Anthony J. Bentley
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