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 Th2:
  a 'nor' b = 'not' (a 'or' b)
proof
  let x be Element of Y;
  thus ('not' (a 'or' b)).x = 'not' (a 'or' b).x by MARGREL1:def 19
      .= 'not' ((a).x 'or' (b).x) by BVFUNC_1:def 4
      .= 'not' 'not' (a.x 'nor' b.x) by BVFUNC_1:53
      .= (a 'nor' b).x by Def4;
end;
