theorem
  M1 is col_circulant & M2 is col_circulant & M3 is col_circulant
  implies a*M1-b*M2+c*M3 is col_circulant
proof
  assume M1 is col_circulant & M2 is col_circulant & M3 is col_circulant;
  then c*M3 is col_circulant & a*M1 - b*M2 is col_circulant by Th20,Th27;
  hence thesis by Th21;
end;
