class TriMatrix
  def initialize
    @store = []
  end
  def [](x,y)
    if x > y
      index = (x*x+x)/2 + y
      @store[index]
    else
      raise IndexError
    end
  end
  def []=(x,y,v)
    if x > y
      index = (x*x+x)/2 + y
      @store[index] = v
    else
      raise IndexError
    end
  end
end

t = TriMatrix.new
t[3,2] = 1
puts t[3,2]  #1
puts t[2,3]  # IndexError

