private static void runString(Environment env, String sourceText, TurtleGraphics t) {
		try {
			LList code = Parser.parse(sourceText);
			Interpreter.init(code, env);
			while(true) {
				// execute until the interpreter is paused
				if (!Interpreter.runUntil(env)) { return; }
				
				// update the display until animation is complete
				while(!t.update()) {
					try { Thread.sleep(1000 / 30); }
					catch(InterruptedException e) {}
				}
			}
		}
		catch(RuntimeError e) {
			System.out.format("runtime error: %s%n", e.getMessage());
			//e.printStackTrace();
			for(LAtom atom : e.trace) {
				System.out.format("\tin %s%n", atom);
			}
			env.reset();
		}
	}