code: public TypeUsage.Builder mapStart(IntUnaryOperator mapper) {
      Objects.requireNonNull(mapper);
      return start(mapper.applyAsInt(start()));
    }
docs: Replaces the value to be returned by {@link TypeUsage#start()} by applying {@code mapper} to it
  and using the result.
  
  @return this {@code Builder} object
  @throws NullPointerException if {@code mapper} is null
  @throws IllegalStateException if the field has not been set