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 Th10:
  a in Line(a,b) & b in Line(a,b)
proof
  a,b,a are_collinear by Th2;
  hence a in Line(a,b);
  a,b,b are_collinear by Th2;
  hence b in Line(a,b);
end;
