Interpolating Registers (The GNU Troff Manual)

#### 5.8.2 Interpolating Registers

Register contents are interpolated with the `\n` escape sequence.

Escape sequence: `\n`i`` ¶
Escape sequence: `\n(`id`` ¶
Escape sequence: `\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