private static NormalSketch asNormal(Sketch<?> sketch) {
        return (NormalSketch) sketch;
    }
Merges one of the two sketches into the other. The merge operation is performed in-place is possible.

@param left Either a {@link NormalSketch} or {@link SparseSketch}.
@param right Either a {@link NormalSketch} or {@link SparseSketch}.
@return The merged sketch. Is either first parameter, the other parameter or a new instance.
searchlib/src/main/java/com/yahoo/searchlib/aggregation/hll/SketchMerger.java