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
  a,b,c is_a_triangle iff for P holds a|'P or b|'P or c|'P by Th5;
