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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 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).