theorem Th13:
  |( p , |[ 0,0,0 ]| )| = 0
  proof
    set e = |[ 0,0,0 ]|;
    e.1 = 0 & e.2 = 0 & e.3 = 0;
    hence |(p,e)| = p.1 * 0 + p.2 * 0 + p.3 * 0 by Lm5
    .= 0;
  end;
