private void addRetPhi(final Subroutine sub, final Block block) {
		if (phis[cfg.preOrderIndex(block)] != null) {
			return;
		}

		final VarExpr target = (VarExpr) prototype.clone();

		final PhiReturnStmt phi = new PhiReturnStmt(target, sub);
		phis[cfg.preOrderIndex(block)] = phi;

		if (SSA.DEBUG) {
			System.out.println("  place " + phi + " in " + block);
		}
	}