  Given a directed graph, G=(V,E), a path query, path(u,v), returns whether
there is a directed path from u to v in G, for u,v vertices in V. Given only V,
exactly learning all the edges in G using path queries is often impossible,
since path queries cannot detect transitive edges. In this paper, we study the
query complexity of exact learning for cases when learning G is possible using
path queries. In particular, we provide efficient learning algorithms, as well
as lower bounds, for multitrees and almost-trees, including butterfly networks.
