theorem
  M1 is line_circulant & M2 is line_circulant implies M1 - M2 is line_circulant
proof
  assume that
A1: M1 is line_circulant and
A2: M2 is line_circulant;
  -M2 is line_circulant by A2,Th11;
  hence thesis by A1,Th7;
end;
