def integral
      left = allocate
      block = Integral.new left, self
      if block.compilable?
        GCCFunction.run block
      else
        block.demand
      end
      left
    end
def integral
      left = allocate
      block = Integral.new left, self
      if block.compilable?
        GCCFunction.run block
      else
        block.demand
      end
      left
    end
Compute integral image

 @return [Node] The integral image of this array.