code: public @NotNull StringAssert excludes(String s) {
      isNotNull();
      if (actual.indexOf(s) == -1) {
        return this;
      }
      failIfCustomMessageIsSet();
      throw failure(format("<%s> should not contain the String:<%s>", actual, s));
    }
docs: Verifies that the actual {@code String} does not contains the given one.
  
  @param s the given {@code String} expected not to be contained in the actual one.
  @return this assertion object.
  @throws AssertionError if the actual {@code String} is {@code null}.
  @throws AssertionError if the actual {@code String} does contain the given one.