let B be B_Lattice; :: thesis: for a, b, c being Element of B st a <=> b = a <=> c holds
b = c

let a, b, c be Element of B; :: thesis: ( a <=> b = a <=> c implies b = c )
set ab = a "/\" b;
set ac = a "/\" c;
set bc = b "/\" c;
set b9c9 = (b `) "/\" (c `);
set a9b9 = (a `) "/\" (b `);
set a9c9 = (a `) "/\" (c `);
set a9b = (a `) "/\" b;
set a9c = (a `) "/\" c;
set ab9 = a "/\" (b `);
set ac9 = a "/\" (c `);
A1: (a <=> b) <=> (a <=> c) = ((a <=> b) "/\" (a <=> c)) "\/" (((a <=> b) `) "/\" ((a <=> c) `)) by Th50;
A2: ( a <=> b = (a "/\" b) "\/" ((a `) "/\" (b `)) & a <=> c = (a "/\" c) "\/" ((a `) "/\" (c `)) ) by Th50;
A3: (a <=> b) ` = (a "/\" (b `)) "\/" ((a `) "/\" b) by Th51;
A4: (a <=> c) ` = (a "/\" (c `)) "\/" ((a `) "/\" c) by Th51;
A5: ((a "/\" b) "\/" ((a `) "/\" (b `))) "/\" ((a "/\" c) "\/" ((a `) "/\" (c `))) = ((a "/\" b) "/\" ((a "/\" c) "\/" ((a `) "/\" (c `)))) "\/" (((a `) "/\" (b `)) "/\" ((a "/\" c) "\/" ((a `) "/\" (c `)))) by LATTICES:def 11;
A6: ( (a "/\" b) "/\" ((a "/\" c) "\/" ((a `) "/\" (c `))) = ((a "/\" b) "/\" (a "/\" c)) "\/" ((a "/\" b) "/\" ((a `) "/\" (c `))) & (a "/\" b) "/\" ((a `) "/\" (c `)) = ((a "/\" b) "/\" (a `)) "/\" (c `) ) by ;
A7: ((a `) "/\" (b `)) "/\" ((a "/\" c) "\/" ((a `) "/\" (c `))) = (((a `) "/\" (b `)) "/\" (a "/\" c)) "\/" (((a `) "/\" (b `)) "/\" ((a `) "/\" (c `))) by LATTICES:def 11;
A8: (b "/\" a) "/\" (a `) = b "/\" (a "/\" (a `)) by LATTICES:def 7;
A9: ((a `) "/\" (b `)) "/\" (a "/\" c) = (((a `) "/\" (b `)) "/\" a) "/\" c by LATTICES:def 7;
A10: ((b `) "/\" (a `)) "/\" a = (b `) "/\" ((a `) "/\" a) by LATTICES:def 7;
A11: ((a "/\" (b `)) "\/" ((a `) "/\" b)) "/\" ((a "/\" (c `)) "\/" ((a `) "/\" c)) = ((a "/\" (b `)) "/\" ((a "/\" (c `)) "\/" ((a `) "/\" c))) "\/" (((a `) "/\" b) "/\" ((a "/\" (c `)) "\/" ((a `) "/\" c))) by LATTICES:def 11;
A12: ( (a "/\" (b `)) "/\" ((a "/\" (c `)) "\/" ((a `) "/\" c)) = ((a "/\" (b `)) "/\" (a "/\" (c `))) "\/" ((a "/\" (b `)) "/\" ((a `) "/\" c)) & (a "/\" (b `)) "/\" ((a `) "/\" c) = ((a "/\" (b `)) "/\" (a `)) "/\" c ) by ;
A13: ((a `) "/\" b) "/\" ((a "/\" (c `)) "\/" ((a `) "/\" c)) = (((a `) "/\" b) "/\" (a "/\" (c `))) "\/" (((a `) "/\" b) "/\" ((a `) "/\" c)) by LATTICES:def 11;
A14: ((b `) "/\" a) "/\" (a `) = (b `) "/\" (a "/\" (a `)) by LATTICES:def 7;
A15: ( b "/\" () = Bottom B & () "/\" (c `) = Bottom B & () "/\" c = Bottom B & a "/\" (a `) = Bottom B & (a `) "/\" a = Bottom B & a "/\" (b `) = (b `) "/\" a & (a `) "/\" b = b "/\" (a `) & ((a `) "/\" b) "/\" (a "/\" (c `)) = (((a `) "/\" b) "/\" a) "/\" (c `) & (b "/\" (a `)) "/\" a = b "/\" ((a `) "/\" a) & ((a "/\" (b `)) "/\" (a "/\" (c `))) "\/" () = (a "/\" (b `)) "/\" (a "/\" (c `)) & () "\/" (((a `) "/\" b) "/\" ((a `) "/\" c)) = ((a `) "/\" b) "/\" ((a `) "/\" c) ) by ;
( (a "/\" b) "/\" (a "/\" c) = ((a "/\" b) "/\" a) "/\" c & (a "/\" b) "/\" a = a "/\" (a "/\" b) & a "/\" (a "/\" b) = (a "/\" a) "/\" b & a "/\" a = a & ((a `) "/\" (b `)) "/\" ((a `) "/\" (c `)) = (((a `) "/\" (b `)) "/\" (a `)) "/\" (c `) & ((a `) "/\" (b `)) "/\" (a `) = (a `) "/\" ((a `) "/\" (b `)) & (a `) "/\" ((a `) "/\" (b `)) = ((a `) "/\" (a `)) "/\" (b `) & (a `) "/\" (a `) = a ` & (a "/\" (b `)) "/\" (a "/\" (c `)) = ((a "/\" (b `)) "/\" a) "/\" (c `) & (a "/\" (b `)) "/\" a = a "/\" (a "/\" (b `)) & (a "/\" b) "/\" c = a "/\" (b "/\" c) & a "/\" (a "/\" (b `)) = (a "/\" a) "/\" (b `) & ((a `) "/\" b) "/\" ((a `) "/\" c) = (((a `) "/\" b) "/\" (a `)) "/\" c & ((a `) "/\" b) "/\" (a `) = (a `) "/\" ((a `) "/\" b) & ((a `) "/\" b) "/\" c = (a `) "/\" (b "/\" c) & (a "/\" (b `)) "/\" (c `) = a "/\" ((b `) "/\" (c `)) & ((a `) "/\" (b `)) "/\" (c `) = (a `) "/\" ((b `) "/\" (c `)) & (a `) "/\" ((a `) "/\" b) = ((a `) "/\" (a `)) "/\" b & ((a "/\" (b "/\" c)) "\/" ((a `) "/\" ((b `) "/\" (c `)))) "\/" ((a "/\" ((b `) "/\" (c `))) "\/" ((a `) "/\" (b "/\" c))) = (((a "/\" (b "/\" c)) "\/" ((a `) "/\" ((b `) "/\" (c `)))) "\/" (a "/\" ((b `) "/\" (c `)))) "\/" ((a `) "/\" (b "/\" c)) & ((a "/\" (b "/\" c)) "\/" ((a `) "/\" ((b `) "/\" (c `)))) "\/" (a "/\" ((b `) "/\" (c `))) = (a "/\" ((b `) "/\" (c `))) "\/" ((a "/\" (b "/\" c)) "\/" ((a `) "/\" ((b `) "/\" (c `)))) & (a "/\" ((b `) "/\" (c `))) "\/" ((a "/\" (b "/\" c)) "\/" ((a `) "/\" ((b `) "/\" (c `)))) = ((a "/\" ((b `) "/\" (c `))) "\/" (a "/\" (b "/\" c))) "\/" ((a `) "/\" ((b `) "/\" (c `))) & (a "/\" ((b `) "/\" (c `))) "\/" (a "/\" (b "/\" c)) = a "/\" (((b `) "/\" (c `)) "\/" (b "/\" c)) & ((b `) "/\" (c `)) "\/" (b "/\" c) = (b "/\" c) "\/" ((b `) "/\" (c `)) & ((a `) "/\" ((b `) "/\" (c `))) "\/" ((a `) "/\" (b "/\" c)) = (a `) "/\" (((b `) "/\" (c `)) "\/" (b "/\" c)) & (Top B) "/\" (((b `) "/\" (c `)) "\/" (b "/\" c)) = ((b `) "/\" (c `)) "\/" (b "/\" c) & ((a "/\" (((b `) "/\" (c `)) "\/" (b "/\" c))) "\/" ((a `) "/\" ((b `) "/\" (c `)))) "\/" ((a `) "/\" (b "/\" c)) = (a "/\" (((b `) "/\" (c `)) "\/" (b "/\" c))) "\/" (((a `) "/\" ((b `) "/\" (c `))) "\/" ((a `) "/\" (b "/\" c))) & a "\/" (a `) = Top B & (a "/\" (((b `) "/\" (c `)) "\/" (b "/\" c))) "\/" ((a `) "/\" (((b `) "/\" (c `)) "\/" (b "/\" c))) = (a "\/" (a `)) "/\" (((b `) "/\" (c `)) "\/" (b "/\" c)) ) by ;
then A16: (a <=> b) <=> (a <=> c) = b <=> c by A2, A1, Th50, A3, A4, A5, A6, A7, A8, A14, A15, A9, A10, A11, A12, A13;
assume A17: a <=> b = a <=> c ; :: thesis: b = c
then A18: (a <=> b) => (a <=> c) = Top B by FILTER_0:28;
A19: b <=> c = Top B by ;
then A20: b "/\" (Top B) [= b "/\" (b => c) by ;
A21: c "/\" (Top B) [= c "/\" (c => b) by ;
A22: b "/\" (b => c) [= c by FILTER_0:def 7;
A23: c "/\" (c => b) [= b by FILTER_0:def 7;
A24: b [= c by ;
c [= b by ;
hence b = c by ; :: thesis: verum