let x be set ; :: thesis: [x,{x}] in [:{x},[x,{x}]:]
[:{x},[x,{x}]:] = {[x,{x}],[x,{x,{x}}]} by Th36;
hence [x,{x}] in [:{x},[x,{x}]:] by TARSKI:def 2; :: thesis: verum