whole_func_string: function(target, vertex) {
        vertex.prev = target.prev;
        vertex.next = target;
  
        if (vertex.prev === null) {
          this.head = vertex;
        } else {
          vertex.prev.next = vertex;
        }
  
        target.prev = vertex;
  
        return this;
      }
func_code_string: function(target, vertex) {
        vertex.prev = target.prev;
        vertex.next = target;
  
        if (vertex.prev === null) {
          this.head = vertex;
        } else {
          vertex.prev.next = vertex;
        }
  
        target.prev = vertex;
  
        return this;
      }
func_documentation_string: Inserts a vertex before the target vertex