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 Th27:
  O_el(Y) 'nor' a = 'not' a
proof
  thus O_el(Y) 'nor' a = 'not' (O_el(Y) 'or' a) by Th2
    .= 'not' a by BVFUNC_1:9;
end;
