theorem Th1:
  {a,b,c} c= A iff a in A & b in A & c in A
  proof
    set X = {a,b,c};
    a in X & b in X & c in X by ENUMSET1:def 1;
    hence {a,b,c} c= A implies a in A & b in A & c in A;
    assume
A1: a in A & b in A & c in A;
    let x;
    thus thesis by A1,ENUMSET1:def 1;
  end;
