reactor-core/src/main/java/reactor/util/function/Tuple2.java
public <R> Tuple2<R, T2> mapT1(Function<T1, R> mapper) {
		return new Tuple2<>(mapper.apply(t1), t2);
	}
public <R> Tuple2<R, T2> mapT1(Function<T1, R> mapper) {
		return new Tuple2<>(mapper.apply(t1), t2);
	}
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