Bisectors of four corners of quadrangle

[Бк] , [Бй] , [^] , [Би]Bisectors of four corners of quadrangle / Always parallelogram(GC World : Various Problems ,QuadrangleБиQuadrangle)

What can you find out about quadrangle EFGH, when quadrangle ABCD is a parallelogram?

Forum of Geometric Constructor by Y.Iijima