theorem
  M1 is col_circulant & M2 is col_circulant implies M1 - M2 is col_circulant
proof
  assume that
A1: M1 is col_circulant and
A2: M2 is col_circulant;
  -M2 is col_circulant by A2,Th25;
  hence thesis by A1,Th21;
end;
