public MeanNumber add(final Number amount, final long count) {
            this.count += count;
            this.sum = NumberHelper.add(sum, mul(amount, count));
            return this;
        }
Advances the starts until a non-null value is found or simply returns {@code null}. In this way, an all
{@code null} stream will result in {@code null}.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MeanGlobalStep.java