reserve F,H,H9 for ZF-formula,
  x,y,z,t for Variable,
  a,b,c,d,A,X for set;
reserve E for non empty set,
  f,g,h for Function of VAR,E,
  v1,v2,v3,v4,v5,u5 for Element of VAL E;

theorem
  for E,f,H,H9 holds E,f |= H 'or' H9 iff E,f |= H or E,f |= H9
proof
  let E,f,H,H9;
  E,f |= 'not' H '&' 'not' H9 iff E,f |= 'not' H & E,f |= 'not' H9 by Th15;
  hence thesis by Th14;
end;
