Support multiple arcs and loops

NOTE: the current version of the website assumes all diagrams will be DAG's

this means adding (p0) -> [t1] -> (p0) doesn't really work - instead it's "last write win so 
effectively it becomes [t1] -> (p0)


Originally I assumed we'd lose some provability w/o DAG's, but it seems the tools currently used in Julia don't have that limitation.