code: func NewCustomMap(lessThan func(l, r interface{}) bool) *SkipList {
  	return &SkipList{
  		lessThan: lessThan,
  		header: &node{
  			forward: []*node{nil},
  		},
  		MaxLevel: DefaultMaxLevel,
  	}
  }
docs: // NewCustomMap returns a new SkipList that will use lessThan as the
  // comparison function. lessThan should define a linear order on keys
  // you intend to use with the SkipList.