let PTN be PT_net_Str ; :: thesis: ({} the Transitions of PTN) *' = {}
consider x being Element of ({} the Transitions of PTN) *' ;
assume not ({} the Transitions of PTN) *' = {} ; :: thesis: contradiction
then x in ({} the Transitions of PTN) *' ;
then ex s being place of PTN st
( x = s & ex f being T-S_arc of PTN ex t being transition of PTN st
( t in {} the Transitions of PTN & f = [t,s] ) ) ;
hence contradiction ; :: thesis: verum