# Get the value at column `y` and row `x`
	#
	# Require: `x >= 0 and x <= width and y >= 0 and y <= height`
	#
	# ~~~
	# var matrix = new Matrix.from([[0.0, 0.1],
	#                               [1.0, 1.1]])
	#
	# assert matrix[0, 0] == 0.0
	# assert matrix[0, 1] == 0.1
	# assert matrix[1, 0] == 1.0
	# assert matrix[1, 1] == 1.1
	# ~~~
	fun [](y, x: Int): Float
	do
		assert x >= 0 and x < width
		assert y >= 0 and y < height

		return items[x + y*width]
	end