A fabric is a pair of policies (fout,fin) used to maintain consistency
* of a packet's virtual and physical locations during virtual compilation.
val generate_fabric : ?log:bool ‑> ?record_paths:string ‑> vrel:Syntax.pred ‑> vtopo:Syntax.policy ‑> ving:Syntax.pred ‑> veg:Syntax.pred ‑> ptopo:Syntax.policy ‑> ping:Syntax.pred ‑> peg:Syntax.pred ‑> fabric