code: function(){
              var isPositive = false,
                  num = this.vertices.length,
                  prev,
                  next,
                  d0,
                  d1,
                  newIsP;
  
              for( var i = 0; i < num; i++ ){
                  prev = (i===0) ? num -1 : i - 1;
                  next = (i===num-1) ? 0 : i + 1;
                  d0 = this.vertices[i].sub(this.vertices[prev]);
                  d1 = this.vertices[next].sub(this.vertices[i]);
                  newIsP = (d0.cross(d1) > 0);
                  if( i === 0 ) {
                      isPositive = true;
                  } else if( isPositive != newIsP ) {
                      return false;
                  }
              }
              return true;
          }
docs: Checks if the polygon is convex.
  @return true, if convex.