code: Definition getDef(String name, Node useNode) {
      checkArgument(getCfg().hasNode(useNode));
      GraphNode<Node, Branch> n = getCfg().getNode(useNode);
      FlowState<MustDef> state = n.getAnnotation();
      return state.getIn().reachingDef.get(allVarsInFn.get(name));
    }
docs: Gets the must reaching definition of a given node.
  
  @param name name of the variable. It can only be names of local variable
  that are not function parameters, escaped variables or variables
  declared in catch.
  @param useNode the location of the use where the definition reaches.