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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.