= Solution
For each requested `x`, enumerate its middle relatives and then each middle value's relatives in the second relation:
``
def compose2[X, Y, Z](r: Rel2[X, Y], s: Rel2[Y, Z]): Rel2[X, Z] =
new Rel2[X, Z] {
def relatives(x: X): List[Z] =
r.relatives(x).flatMap(y => s.relatives(y)).distinct
}
``
A returned `z` is present exactly when some enumerated `y` witnesses the two relation memberships. Conversely, every such witness appears in `r.relatives(x)`, and its `z` appears in `s.relatives(y)`, so no composed relative is omitted. `.distinct` removes repeated endpoints, including repetitions produced through different witnesses. \b[This is the same <composition of relations> as part (b), represented one queried row at a time.] Neither the entire first carrier nor the entire result must be enumerated in advance. The relation operations must remain consistent while the returned view is used; changing their underlying state would make this a live view rather than a snapshot.
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