question: public HalfEdge addEdge ( Coordinate orig , Coordinate dest ) { if ( ! isValidEdge ( orig , dest ) ) return null ; HalfEdge eAdj = ( HalfEdge ) vertexMap . get ( orig ) ; HalfEdge eSame = null ; if ( eAdj != null ) { eSame = eAdj . find ( dest ) ; } if ( eSame != null ) { return eSame ; } HalfEdge e = insert ( orig , dest , eAdj ) ; return e ; }
target:  Adds an edge between the coordinates orig and dest to this graph. Only valid edges can be added (in particular, zero-length segments cannot be added)