code: public <R> Tuple2<R, T2> mapT1(Function<T1, R> mapper) {
  		return new Tuple2<>(mapper.apply(t1), t2);
  	}
docs: Map the left-hand part (T1) of this {@link Tuple2} into a different value and type,
  keeping the right-hand part (T2).
  
  @param mapper the mapping {@link Function} for the left-hand part
  @param <R> the new type for the left-hand part
  @return a new {@link Tuple2} with a different left (T1) value