private void resetDownSafe(final Def def) {
		if (StackPRE.DEBUG) {
			System.out.println("        reset down safe for " + def);
		}

		if (def instanceof Phi) {
			final Phi phi = (Phi) def;

			if (phi.downSafe()) {
				phi.setDownSafe(false);

				final Iterator e = phi.operands().iterator();

				while (e.hasNext()) {
					final Def operand = (Def) e.next();

					if (operand != null) {
						resetDownSafe(operand);
					}
				}
			}
		} else {
			def.setDownSafe(false);
		}
	}