let c, x, y be set ; :: thesis: ( c <> [<*x,y*>,'or' ] implies for s being State of (2GatesCircuit x,y,c,'or' )
for a1, a2, a3 being Element of BOOLEAN st a1 = s . x & a2 = s . y & a3 = s . c holds
(Following s,2) . (2GatesCircOutput x,y,c,'or' ) = (a1 'or' a2) 'or' a3 )
set f = 'or' ;
assume A1: c <> [<*x,y*>,'or' ] ; :: thesis: for s being State of (2GatesCircuit x,y,c,'or' )
for a1, a2, a3 being Element of BOOLEAN st a1 = s . x & a2 = s . y & a3 = s . c holds
(Following s,2) . (2GatesCircOutput x,y,c,'or' ) = (a1 'or' a2) 'or' a3
let s be State of (2GatesCircuit x,y,c,'or' ); :: thesis: for a1, a2, a3 being Element of BOOLEAN st a1 = s . x & a2 = s . y & a3 = s . c holds
(Following s,2) . (2GatesCircOutput x,y,c,'or' ) = (a1 'or' a2) 'or' a3
let a1, a2, a3 be Element of BOOLEAN ; :: thesis: ( a1 = s . x & a2 = s . y & a3 = s . c implies (Following s,2) . (2GatesCircOutput x,y,c,'or' ) = (a1 'or' a2) 'or' a3 )
assume ( a1 = s . x & a2 = s . y & a3 = s . c ) ; :: thesis: (Following s,2) . (2GatesCircOutput x,y,c,'or' ) = (a1 'or' a2) 'or' a3
hence (Following s,2) . (2GatesCircOutput x,y,c,'or' ) = 'or' . <*('or' . <*a1,a2*>),a3*> by A1, Th62
.= 'or' . <*(a1 'or' a2),a3*> by Def5
.= (a1 'or' a2) 'or' a3 by Def5 ;
:: thesis: verum