code: public static <T extends Number> ErrorMessageFactory shouldBeEqual(T actual, T expected, Offset<T> offset, T difference) {
      return new ShouldBeEqualWithinOffset(actual, expected, offset, difference);
    }
docs: Creates a new <code>{@link ShouldBeEqualWithinOffset}</code>.
  @param <T> guarantees that the values used in this factory have the same type.
  @param actual the actual value in the failed assertion.
  @param expected the expected value in the failed assertion.
  @param offset the given positive offset.
  @param difference the effective difference between actual and expected.
  @return the created {@code ErrorMessageFactory}.