    XCTAssertEqual("".stp_safeSubstring(to: 0), "")
    XCTAssertEqual("".stp_safeSubstring(to: 1), "")
  }

  func testSafeSubstringFromIndex() {
    XCTAssertEqual("foo".stp_safeSubstring(from: 0), "foo")
    XCTAssertEqual("foo".stp_safeSubstring(from: 1), "oo")
    XCTAssertEqual("foo".stp_safeSubstring(from: 3), "")
    XCTAssertEqual("".stp_safeSubstring(from: 0), "")
    XCTAssertEqual("".stp_safeSubstring(from: 1), "")
  }