Zvonko Cerin, On butterflies inscribed in a quadrilateral,
Forum Geometricorum, 6 (2006) 241--246.
Abstract: We explore a configuration consisting of two quadrilaterals
which share the intersection of diagonals. We prove results analogous to
the Sidney Kung's Butterfly Theorem for Quadrilaterals in Kung,
Mathematics Magazine, 78 (2005) 314--316.