reserve G for IncProjStr;
reserve a,a1,a2,b,b1,b2,c,d,p,q,r for POINT of G;
reserve A,B,C,D,M,N,P,Q,R for LINE of G;

theorem
  (ex a,b,c,d st a,b,c,d is_a_quadrangle) implies ex a,b,c st a,b,
  c is_a_triangle;
