reserve k,m,n for Element of NAT,
  a,X,Y for set,
  D,D1,D2 for non empty set;
reserve p,q for FinSequence of NAT;
reserve x,y,z,t for Variable;

theorem
  a is ZF-formula iff a in WFF
proof
  thus a is ZF-formula implies a in WFF
  proof
    assume a is ZF-formula;
    then a is Element of WFF by Def9;
    hence thesis;
  end;
  assume a in WFF;
  hence thesis by Def8,Def9;
end;
