Register contents are interpolated with the \n escape sequence.
\ni\n(id\n[ident] ¶Interpolate register with name ident (one-character
name i, two-character name id).  \n is
interpreted even in copy mode (see Copy Mode).  If the register is
undefined, it is created and assigned a value of ‘0’, that
value is interpolated, and a warning in category ‘reg’ is emitted.
See Warnings, for information about the enablement and suppression of
warnings.
.nr a 5
.nr as \na+\na
\n(as
    ⇒ 10
.nr a1 5
.nr ab 6
.ds str b
.ds num 1
\n[a\n[num]]
    ⇒ 5
\n[a\*[str]]
    ⇒ 6