Changed add and del (now remove) functions according to signatures
described in architecture doc.
addEdge checks for existence of nodes
removeNode: checks if some edge has node as it's source/target before
removing it
IntMap imported qualified (except the IntMap Type itself)
Implements morphism application for digraphs
Merge remote-tracking branch 'origin'
Add keep actions for Digraphs
Fix issue with incomplete pattern matching
Digraph module now exports Node and Edge constructors; started and example file for tests
Merge branch 'master' of github.com:rodrigo-machado/ufrgs-grt