theorem Th14: :: PROJPL_1:14

for G being IncProjStr
for a1, a2, b1, b2 being POINT of G
for A, B being LINE of G st G is configuration & {a1,a2} on A & {b1,b2} on B & a1,a2 |' B & b1,b2 |' A & a1 <> a2 & b1 <> b2 holds
a1,a2,b1,b2 is_a_quadrangle