@Nonnull
	public static String fillLeft(int len, @Nonnull String pattern, @Nullable String string) {
		StringBuilder sb = new StringBuilder(string == null ? "" : string);
		while (sb.length() < len)
			sb.insert(0, pattern);
		return sb.toString();
	}
@Test
	void testFillLeft() {
		assertEquals("aaab", StringUtil.fillLeft(4, "a", "b"));
		assertEquals("aaaa", StringUtil.fillLeft(4, "a", null));
	}