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