Leading Space Traps (The GNU Troff Manual)

#### 5.28.4 Leading Space Traps

Request: `.lsm` [name] ¶
Register: `\n[lsn]` ¶
Register: `\n[lss]` ¶

  Set a leading space trap, calling the macro name when GNU `troff` encounters leading spaces in an input line; the implicit line break that normally happens in this case is suppressed. If no argument is supplied, the default leading space behavior is (re-)established (see Breaking).

  The count of leading spaces on an input line is stored in register `lsn`, and the amount of corresponding horizontal motion in register `lss`, irrespective of whether a leading space trap is set. When it is, the leading spaces are removed from the input line, and no motion is produced before calling name.