# 33-4 Picking up sticks

Professor Charon has a set of $n$ sticks, which are piled up in some configuration. Each stick is specified by its endpoints, and each endpoint is an ordered triple giving its $(x, y, z)$ coordinates. No stick is vertical. He wishes to pick up all the sticks, one at a time, subject to the condition that he may pick up a stick only if there is no other stick on top of it.

a. Give a procedure that takes two sticks $a$ and $b$ and reports whether $a$ is above, below, or unrelated to $b$.

b. Describe an efficient algorithm that determines whether it is possible to pick up all the sticks, and if so, provides a legal order in which to pick them up.

(Omit!)