# File narray_ext.rb, line 265
  def rot90(n_times=1)
    if self.rank < 2
      raise "must be >= 2 dimensional array"
    end
    case n_times%4
    when 0
      self.dup
    when 1
      self.transpose(1,0).reverse(0)
    when 2
      self.reverse(0,1)
    when 3
      self.transpose(1,0).reverse(1)
    end
  end