gtroff reads in the text for a macro, string, or diversion,
it copies the text (including request lines, but excluding escapes) into
an internal buffer. Escapes are converted into an internal form, except
which are evaluated and inserted into the text where the escape was
located. This is known as copy-in mode or copy mode.
What this means is that you can specify when these escapes are to be
evaluated (either at copy-in time or at the time of use) by insulating
the escapes with an extra backslash. Compare this to the
\edef commands in TeX.
The following example prints the numbers 20 and 10:
.nr x 20 .de y .nr x 10 \&\nx \&\\nx .. .y