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