`type graph`
`val graph : ?loops:bool -> xrange:(int * int) -> yrange:(int * int) -> prob:float -> int -> graph`

`graph xrange yrange prob v` generates a random planar graph with exactly `v` vertices. Vertices are labeled with integer coordinates, randomly distributed according to `xrange` and `yrange`. Edges are built as follows: the full Delaunay triangulation is constructed and then each edge is discarded with probabiblity `prob` (which should lie in `0..1`). In particular `prob = 0.0` gives the full triangulation. Edges are labeled with the (rounded) Euclidean distance between the two vertices. The boolean `loops` indicates whether loops are allowed; default value is no loop (`false`).