
theorem
  for G1, G2 being _Graph, S being GraphSum of G1, G2
  holds S.size() = G1.size() +` G2.size()
proof
  let G1, G2 be _Graph, S be GraphSum of G1, G2;
  S is GraphSum of <* G1, G2 *> by Def28;
  then consider G9 being GraphUnion of rng canGFDistinction(<* G1, G2 *>)
    such that A1: S is G9-Disomorphic by Def27;
  consider H being PGraphMapping of G9, S such that
    A2: H is Disomorphism by A1, GLIB_010:def 24;
  consider G3, G4 being _Graph such that
    A3: the_Edges_of G3 misses the_Edges_of G4 and
    A4: G3 tolerates G4 and
    the_Vertices_of G3 misses the_Vertices_of G4 and
    A5: G9 is GraphUnion of G3, G4 and
    A6: G3 is G1-Disomorphic & G4 is G2-Disomorphic by Lm6;
  consider H1 being PGraphMapping of G1, G3 such that
    A7: H1 is Disomorphism by A6, GLIB_010:def 24;
  consider H2 being PGraphMapping of G2, G4 such that
    A8: H2 is Disomorphism by A6, GLIB_010:def 24;
  thus S.size() = G9.size() by A2, GLIB_010:84
    .= G3.size() +` G4.size() by A3, A4, A5, GLIBPRE1:121
    .= G1.size() +` G4.size() by A7, GLIB_010:84
    .= G1.size() +` G2.size() by A8, GLIB_010:84;
end;
