reserve x,y for set;
reserve X for non empty set;
reserve a,b,c,d for Element of X;
reserve S for OAffinSpace;
reserve a,b,c,d,p,q,r,x,y,z,t,u,w for Element of S;

theorem Th31:
  x,x,y are_collinear & x,y,y are_collinear & x,y,x are_collinear
          by Th19,Th20;
