🚀 go-pugleaf

A psroff user writes:
| .EQ
| left nothing {d f(x)} over {d x} right |
| .EN

| I get 
| 
|                         df(x) /
|                         ----- /
|                           dx  /

Eqn emits:
| .EQ
| .nr 99 \n(.s
| .nr 98 \n(.f
| .ps 10
| .ft 2
| .ds 11 "d
| .ds 12 "\^f\|\|\f1(\fPx\|\f1)\fP
| .as 11 "\*(12
| .ds 12 "d
| .ds 13 "x
| .as 12 "\*(13
| .nr 11 \w'\s10\*(11'
| .nr 12 \w'\s10\*(12'
| .nr 13 \n(11
| .if \n(12>\n(13 .nr 13 \n(12
| .nr 13 \n(13+\s10.5m\s0
| .ds 11 \v'42u'\h'\n(13u-\n(12u/2u'\*(12\
| \h'-\n(12u-\n(11u/2u'\v'-78u'\*(11\
| \h'-\n(13u-\n(11u/2u+.1m'\v'18u'\l'\n(13u-.2m'\h'.1m'\v'18u'
| .ds 11 \|\v'-3u'\v'3u'\*(11\|\v'-3u'\b'|||'\v'3u'
                                      ^^^^^^^
| .ds 11 \x'0'\x'0-45u'\f2\s10\*(11\s\n(99\f\n(98\x'51u'
| .nr 11 \w'\*(11'
| .nr MK 0
| .if 180>\n(.v .ne 180u
| .rn 11 10
| \*(10
| .ps \n(99
| .ft \n(98
| .EN

By my reckoning, the "pile" of "|" is in font 2 (Italic).  Psroff agrees,
and emits a request for the "|" character in the Times-Italic font which
is slanted (in Postscript).

To me this looks like a bug in eqn.  On the other hand, pscat seems to
emit vertical "|" in this example.  I'm not sure whether pscat thinks
it's in font 1 or 2, or whether this is special cased.

I can fix this in psroff simply enough (have *all* "|" come out
in Times-Roman), but this doesn't seem quite right to me.

Any idears?