reserve X for set;
reserve CS for non empty CollStr;
reserve a,b,c for Point of CS;
reserve CLSP for CollSp;
reserve a,b,c,d,p,q,r for Point of CLSP;

theorem Th8:
  a,b,c are_collinear implies b,c,a are_collinear
proof
  assume a,b,c are_collinear;
  then b,a,c are_collinear by Th4;
  hence thesis by Th4;
end;
