private fun rotate_l90_face(face_id: Int) do
		var lines = r90_cache
		if lines.is_empty then for i in [0 .. 3[ do lines.add(new Array[Int])
		array1d_copy_to(faces[face_id][top_ln], lines[0])
		array1d_copy_to(faces[face_id][mid_ln], lines[1])
		array1d_copy_to(faces[face_id][bottom_ln], lines[2])
		for i in [0 .. 3[ do lines[i].swap_at(0, 2)
		set_coldata(face_id, left_col, lines[0])
		set_coldata(face_id, mid_col, lines[1])
		set_coldata(face_id, right_col, lines[2])
	end