boolean incremental =
(oldWit.bits & WIT.CELL) == (wit.bits & WIT.CELL) // same cell type
&& ((oldWit.bits & WIT.PURE_MARK) == 0 || (wit.bits & WIT.SOLID) == 0)
&& (oldWit.bits & ~wit.bits) == 0 // no marks removed
&& (oldWit.bits & WIT.OVER_CELL) == 0 // no over-cell markup on screen (like dimmed)
&& oldWit.sameLabel(wit); // same label
if (incremental)
bits &= ~oldWit.bits;
oldWit.set(wit);
//#ifdef debug