theorem Th13: :: PROJPL_1:13

for G being IncProjStr
for a, b, c, d being POINT of G st a,b,c,d is_a_quadrangle holds
( 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;