[Heirloom] Using the Symbola font in Heirloom troff

[T. Kurt Bond]

(The heirloom-doctools README.md
<https://github.com/n-t-roff/heirloom-doctools/blob/master/README.md> says
to ask Heirloom doctools questions on this list.)

I'd like to use the Symbola font in Heirloom troff.   I tried the following:

.do xflag 3
.\" fp 5 Optima Optima-Regular ttf
.fp 5 Symbola Symbola otf
.LP
Here is some normal text.
.\" PLAYING CARD ACE OF SPACES is Unicode 0x1F0A1
.ft Symbola
🂡 And some normal text. ❊
.ft P
More normal text.

That's a literal PLAYING CARD ACE OF SPADES Unicode character at the start
of the line between the two .ft requests.  That character does not show up
in the troff output, even through the EIGHT TEARDROP-SPOKED PROPELLER
ASTERISK Unicode character at the end of the line *does* show up,
as CPSuni274A where the CPS<name> outputs the character of that name.  The
Symbola font is embedded in the PDF output (created from the PostScript
output), and the text "And some normal text" and the EIGHT TEARDROP-SPOKED
PROPELLER ASTERISK Unicode character are in the Symbola font in the troff
output.

However, if I manually add a CPSuni1F0A1 to the troff output, *that* character
*does* show up.

Any ideas as to why the literal PLAYING CARD ACE OF SPADES Unicode
character in the document source is being ignored and not written to the
troff output?

I actually have a document that needs to use the PLAYING CARD ACE OF SPADES
Unicode character.  The ultimate goal is to have the Symbola font used as a
fallback font, which should happen automatically in Heirloom troff, since
it searches all the fonts when a font is missing a character, but I made
the example use the Symbola font directly because that shows the problem
directly.

-- 
T. Kurt Bond, tkurtbond@gmail.com, https://tkurtbond.github.io