reserve Y for non empty set,
  a,b,c,d for Function of Y,BOOLEAN;
reserve Y for non empty set,
  a,b,c for Function of Y,BOOLEAN;

theorem Th3:
  I_el(Y) 'nand' a = 'not' a
proof
  I_el(Y) 'nand' a = 'not' (I_el(Y) '&' a) by th1;
  hence thesis by BVFUNC_1:6;
end;
