theorem Th23: :: PROJPL_1:23

for G being IncProjStr holds
( G is IncProjectivePlane iff ( G is configuration & ( for p, q being POINT of G ex M being LINE of G st {p,q} on M ) & ( for P, Q being LINE of G ex a being POINT of G st a on P,Q ) & ex a, b, c, d being POINT of G st a,b,c,d is_a_quadrangle ) )