theorem Th27: :: PARSP_2:27

for FdSp being FanodesSp
for a, b, c, d being Element of FdSp st parallelogram a,b,c,d holds
( not a,b,c are_collinear & not b,a,d are_collinear & not c,d,a are_collinear & not d,c,b are_collinear )