Chapter 3.5.1: Exercise 3.50 "optimized" solution function is incorrect

[PossibilityZero]

The current solution to Exercise 3.50 is given as follows:

function stream_map_2(f, s1, s2) {
    return is_null(s1) || is_null(s2)
        ? null
        : pair(f(head(s1), head(s2)),
                () => stream_map_2(f, stream_tail(s1), stream_tail(s2)));
}

function stream_map_2_optimized(f, s1, s2) {
    return is_null(s1) || is_null(s2)
        ? null
        : pair(f(head(s1), head(s2)),
                memo(() => stream_map_2(f, stream_tail(s1), stream_tail(s2))));
}

The function stream_map_2_optimized as given only uses memoization for the first pair, because it calls stream_map_2. The last line should be changed to

                memo(() => stream_map_2_optimized(f, stream_tail(s1), stream_tail(s2))));

[martin-henz]

OK, fixed in #1062.