Interactive DAG

An (not-yet) efficient incremental DAG for real-time type verification systems or other purposes.

Maintaining the topological ordering is based on the work presented in this paper:

A Dynamic Topological Sort Algorithm for Directed Acyclic Graphs
   DAVID J. PEARCE / PAUL H. J. KELLY
   Journal of Experimental Algorithmics (JEA)
   Volume 11, 2006, Article No. 1.7
   ACM New York, NY, USA