question: public QuadEdge connect ( QuadEdge a , QuadEdge b ) { QuadEdge q = QuadEdge . connect ( a , b ) ; quadEdges . add ( q ) ; return q ; }
target:  Creates a new QuadEdge connecting the destination of a to the origin of b, in such a way that all three have the same left face after the connection is complete. The quadedge is recorded in the edges list.