This applet utilizes the Cassowary solver. It
demonstrates the use of both equality and inequality constraints. To see it work,
try dragging any of the line on the diagram.
The constraints involved require that all the lines
are within the boundary of the applet, and each end-point of a line is attached to
end-point of another line within the same quadrilateral. The corners of the inner
quadrilateral are constrained to the mid-points of the outer quadrilateral.