What does AP save during evolution? Terminology.
Diff(TG1, TG2), Diff(T(G1, S1), T(G2, S2)): list differences for each node.
Errors(G2,T(G1,S1))): compiler error messages
Errors(G2, G1): if G1 has an edge (A,B) then G2 has such an edge, otherwise report error.