[Bug 46441] sh(1): Does not support PS1, PS2, PS4 parameter expansion

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=46441

--- Comment #12 from Jilles Tjoelker <jilles@FreeBSD.org> ---
For PS4, I implemented this in 2011. The problem with unbounded recursion from
command substitutions is avoided by not allowing command substitutions; this
error (like other parse and expansion errors in PS4) is handled by writing an
error message and continuing with the unexpanded value.

For PS1 and PS2, there are two additional problems:
* how does $... expansion interact with the existing \u, \w, \$, etc.
* because PS2 is needed during the middle of a parse, more work may be needed
to prevent the two parse operations from interfering with each other

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