let G1, G2 be Graph; ( the Source of G1 tolerates the Source of G2 & the Target of G1 tolerates the Target of G2 implies ( G1 c= G1 \/ G2 & G2 c= G1 \/ G2 ) )
assume
( the Source of G1 tolerates the Source of G2 & the Target of G1 tolerates the Target of G2 )
; ( G1 c= G1 \/ G2 & G2 c= G1 \/ G2 )
then
G1 \/ G2 is_sum_of G1,G2
;
hence
( G1 c= G1 \/ G2 & G2 c= G1 \/ G2 )
by Th18; verum