collapseos/blk/454

( handle backspace: go back one char in IN>, if possible, then
  emit SPC + BS )
: (inbs)
    ( already at IN( ? )
    IN> @ IN( = IF EXIT THEN
    IN> @ 1- IN> !
    SPC BS
;






( cont.: read one char into input buffer and returns whether we
  should continue, that is, whether CR was not met. )