tree vs graph

in tree, every node has a single predecessor, could have multiple sucessors.
in graph, there is no limitation, multiple predecessors, multiple sucessors.

RPO in graph, ensures all predecessors are visited before the current node is visited. Topologic sorting, forward data flow analysis.
RPO is the same as pre-order in tree, but different in graph.

post-order ensures all sucessors are visited before current node is visited. This is the reverse of RPO. backward data analysis.

forward data flow analysis: reaching definition
backward data flow analysis: live variable analysis.