defpred S₁[ set ] means ex k being Nat st
( k <= 1 & $1 = A |^ k );
reconsider M = { B where B is Subset of : S₁[B] } as Subset-Family of from DOMAIN_1:sch 7();
union M is Subset of ;
hence union { B where B is Subset of : ex k being Nat st
( k <= 1 & B = A |^ k ) } is Subset of ; :: thesis: verum