[groff] Heirloom troff invoke macro from string

[Mircea Hossu]

Is it possible in Heirloom troff to invoke a macro from a string?

There is an escape sequence:
  \R'R ±N' => Set number register R to ±N

This is useful, but the expression is too limiting in some cases. For
example, is it possible to increase register R by 1 only if it hasn't
reached a maximum value?

There is also an output line trap escape sequence:
  \Px,\P(xx,\P[xxx] => Output-line trap x, xx, or xxx

This is very useful (and very close), but there are situations where
one would need for that macro to be invoked at that *exact* location, as
opposed to later, when the text is printed.

Thank you!