Implement a function that recurses from n down to 1, reporting every level to on_enter on the way down and to on_leave on the way back up.

• descend(n, on_enter, on_leave) calls on_enter(n), then calls itself with n - 1, then calls on_leave(n). The order of those three steps is the exercise.
• The recursion stops below 1. When n is 0 or negative, descend returns immediately and calls nothing at all.
• Each on_leave(n) runs only after the nested call has returned, so the leave calls come out in the reverse order of the enter calls. descend(3, ...) enters 3, 2, 1, then leaves 1, 2, 3.
• The value handed to a callback is the current n. The first on_enter receives the n you were called with, the last one receives 1.
• Either callback may be NULL. Skip that call and keep recursing: descend(4, on_enter, NULL) still walks all four levels, it only stays silent on the way up. With both pointers NULL the call produces no output and still has to return.
• descend prints nothing on its own. Every byte of output comes from the callbacks you were handed. The examples use a pair that prints enter k and leave k, one per line.
• Graded calls keep n at 10 or below, so plain recursion is enough.

Implement a function that recurses from n down to 1, reporting every level to on_enter on the way down and to on_leave on the way back up.

• descend(n, on_enter, on_leave) calls on_enter(n), then calls itself with n - 1, then calls on_leave(n). The order of those three steps is the exercise.
• The recursion stops below 1. When n is 0 or negative, descend returns immediately and calls nothing at all.
• Each on_leave(n) runs only after the nested call has returned, so the leave calls come out in the reverse order of the enter calls. descend(3, ...) enters 3, 2, 1, then leaves 1, 2, 3.
• The value handed to a callback is the current n. The first on_enter receives the n you were called with, the last one receives 1.
• Either callback may be NULL. Skip that call and keep recursing: descend(4, on_enter, NULL) still walks all four levels, it only stays silent on the way up. With both pointers NULL the call produces no output and still has to return.
• descend prints nothing on its own. Every byte of output comes from the callbacks you were handed. The examples use a pair that prints enter k and leave k, one per line.
• Graded calls keep n at 10 or below, so plain recursion is enough.