.lsm [name] ¶\n[lsn] ¶\n[lss] ¶Set a leading space trap, calling the macro name when GNU
troff encounters leading spaces on a text line; the implicit
line break that normally happens in this case is suppressed. The
formatter stores the count of leading spaces on the text line 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, GNU troff removes the leading spaces from
the input line and produces no motion before calling name.
If no argument is supplied, GNU troff reëstablishes the
default handling of leading spaces on text lines (breaking the line when
filling, and formatting a horizontal motion of ‘\n[lsn]’ word
spaces).