public EdgeType getEdgeType(BaseWork a, BaseWork b) {
    return edgeProperties.get(new ImmutablePair(a,b)).getEdgeType();
  }
remove removes a node from the graph and removes all edges with
work as start or end point. No change to the graph if the node
doesn't exist.
ql/src/java/org/apache/hadoop/hive/ql/plan/TezWork.java