code: function(elem, props, delta, vector) {
            var v = vector.scale(-delta);  
            if(v.norm() < 1) {
                var x = v.x, y = v.y;
                var ans = elem.startPos
                  .getc().moebiusTransformation(v);
                elem.pos.setc(ans.x, ans.y);
                v.x = x; v.y = y;
              }           
          }
docs: Position interpolators