reserve X for non empty CUNITSTR;
reserve a, b for Complex;
reserve x, y for Point of X;
reserve X for ComplexUnitarySpace;
reserve x, y, z, u, v for Point of X;

theorem
  x, y are_orthogonal implies -x, y are_orthogonal
proof
  assume x, y are_orthogonal;
  then - x .|. y = - 0;
  hence thesis by Th18;
end;
