public void addPhi(final Block block) {
			final int blockIndex = cfg.preOrderIndex(block);

			if (phis[blockIndex] == null) {
				if (SSAPRE.DEBUG) {
					System.out.println("    add phi for " + prototype + " at "
							+ block);
				}

				phis[blockIndex] = new Phi(this, block);
			}
		}