theorem Th34:
  a 'nor' (b 'or' c) = 'not' (a 'or' b 'or' c)
proof
  thus a 'nor' (b 'or' c) = 'not' (a 'or' (b 'or' c)) by Th2
    .= 'not' (a 'or' b 'or' c) by BVFUNC_1:8;
end;
