get_partial_index Function

  function get_partial_index(this, upstream_grad) result(output)
    implicit none
    class(array_type), intent(inout) :: this
    type(array_type), intent(in) :: upstream_grad
    type(array_type) :: output
    type(array_type), pointer :: ptr

    ptr => reverse_index( &
         upstream_grad, indices=this%indices, from=.false., &
         new_index_size=size(this%left_operand%val, 2) &
    )
    call output%assign_and_deallocate_source(ptr)
  end function get_partial_index