code: @CanIgnoreReturnValue
    public V erase(@Nullable Object rowKey, @Nullable Object columnKey) {
      Integer rowIndex = rowKeyToIndex.get(rowKey);
      Integer columnIndex = columnKeyToIndex.get(columnKey);
      if (rowIndex == null || columnIndex == null) {
        return null;
      }
      return set(rowIndex, columnIndex, null);
    }
docs: Associates the value {@code null} with the specified keys, assuming both keys are valid. If
  either key is null or isn't among the keys provided during construction, this method has no
  effect.
  
  <p>This method is equivalent to {@code put(rowKey, columnKey, null)} when both provided keys
  are valid.
  
  @param rowKey row key of mapping to be erased
  @param columnKey column key of mapping to be erased
  @return the value previously associated with the keys, or {@code null} if no mapping existed
  for the keys