# Subtexture for the tile at `x, y`
	#
	# Require: `x < nb_cols and y < nb_rows`
	fun [](x,y: Int): Texture
	do
		assert x >= 0 and x < nb_cols and y >= 0 and y <= nb_rows else print "{x}x{y}<?{nb_cols}x{nb_rows}"
		var idx = x + y * nb_cols
		return subtextures[idx]
	end