theorem
  L1 is C_Linear_Combination of A & L2 is C_Linear_Combination of A
  implies L1 - L2 is C_Linear_Combination of A
proof
  assume that
A1: L1 is C_Linear_Combination of A and
A2: L2 is C_Linear_Combination of A;
  - L2 is C_Linear_Combination of A by A2,Th26;
  hence thesis by A1,Th20;
end;
