theorem
  p is first-symmetry-of-circulant & q is first-symmetry-of-circulant & len p =
  len q implies a*(SCirc p)+b*(SCirc q)=SCirc(a*p+b*q)
proof
  set n = len p;
  assume that
A1: p is first-symmetry-of-circulant and
A2: q is first-symmetry-of-circulant and
A3: len p = len q;
A4: a*p is first-symmetry-of-circulant & b*q is first-symmetry-of-circulant
       by A1,A2,Th12;
A5: len (b*q)=n by A3,MATRIXR1:16;
  a*(SCirc p)+b*(SCirc q) = SCirc (a*p)+b*(SCirc q) by A1,Th13
    .=SCirc (a*p)+SCirc (b*q) by A2,Th13
    .=SCirc(a*p+b*q) by A4,A5,Th11,MATRIXR1:16;
  hence thesis;
end;
