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 Th13:
  a,b,c,d is_a_quadrangle implies a,c,b,d is_a_quadrangle & b,a,c,
d is_a_quadrangle & b,c,a,d is_a_quadrangle & c,a,b,d is_a_quadrangle & c,b,a,d
  is_a_quadrangle & a,b,d,c is_a_quadrangle & a,c,d,b is_a_quadrangle & b,a,d,c
  is_a_quadrangle & b,c,d,a is_a_quadrangle & c,a,d,b is_a_quadrangle & c,b,d,a
  is_a_quadrangle & a,d,b,c is_a_quadrangle & a,d,c,b is_a_quadrangle & b,d,a,c
  is_a_quadrangle & b,d,c,a is_a_quadrangle & c,d,a,b is_a_quadrangle & c,d,b,a
  is_a_quadrangle & d,a,b,c is_a_quadrangle & d,a,c,b is_a_quadrangle & d,b,a,c
  is_a_quadrangle & d,b,c,a is_a_quadrangle & d,c,a,b is_a_quadrangle & d,c,b,a
  is_a_quadrangle
by Th11;
