reserve x, y, z, s for ExtReal;
reserve i, j for Integer;
reserve n, m for Nat;
reserve x, y, v, u for ExtInt;

theorem Th12:
  for K being distributive left_unital add-associative right_zeroed
  right_complementable non empty doubleLoopStr holds
  (-1.K) * (-1.K) = 1.K
  proof
    let K be distributive left_unital add-associative right_zeroed
    right_complementable non empty doubleLoopStr;
    thus (-1.K) * (-1.K) = 1.K * 1.K by VECTSP_1:10
    .= 1.K;
  end;
