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 implies a,c,b is_a_triangle & b,a,c is_a_triangle
  & b,c,a is_a_triangle & c,a,b is_a_triangle & c,b,a is_a_triangle by Th11;
