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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.