ReadonlypieceReadonlyremainderThe part of the element inside NO zone could not be built.
Separate from sumErrorRel because the two have opposite fixes: a
raised sum means the zones overlap and want redrawing, while this means
real volume is MISSING from the result. A caller must refuse the split
outright rather than publish the zone pieces alone.
ReadonlysumHow far the pieces are from summing to the whole, relative.
The invariant #2508 puts above everything else here. Exposed rather than enforced: the caller decides what to do with a split that does not add up (the expected cause is zones that overlap each other), and a number it can show beats a silent refusal.
ReadonlywholeEnclosed volume of the input element.
Piece index, or undefined when out of range. Each call COPIES the
piece out, matching the rest of this API surface.
The result of splitting one element.