increase(position, value) {
    if (position < 1 || position > this.arraySize) {
      throw new Error('Position is out of allowed range');
    }

    for (let i = position; i <= this.arraySize; i += (i & -i)) {
      this.treeArray[i] += value;
    }

    return this;
  }
Adds value to existing value at position.

@param  {number} position
@param  {number} value
@return {FenwickTree}