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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
     (*note 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.