reserve p,p1,p2,q,r,F,G,G1,G2,H,H1,H2 for ZF-formula,
  x,x1,x2,y,y1,y2,z,z1,z2,s,t for Variable,
  a,X for set;
reserve M for non empty set,
  m,m9 for Element of M,
  v,v9 for Function of VAR,M;
reserve i,j for Element of NAT;

theorem
  WFF c= bool [:NAT,NAT:]
proof
  let a be object;
  assume a in WFF;
  then reconsider H = a as ZF-formula by ZF_LANG:4;
  H c= [:NAT,NAT:];
  hence thesis;
end;
