function(a, b) {
      if (a.prev === null) {
        this.head = b.next;
      } else {
        a.prev.next = b.next;
      }

      if (b.next === null) {
        this.tail = a.prev;
      } else {
        b.next.prev = a.prev;
      }

      return this;
    }
function(a, b) {
      if (a.prev === null) {
        this.head = b.next;
      } else {
        a.prev.next = b.next;
      }

      if (b.next === null) {
        this.tail = a.prev;
      } else {
        b.next.prev = a.prev;
      }

      return this;
    }
function ( a , b ) { if ( a . prev === null ) { this . head = b . next ; } else { a . prev . next = b . next ; } if ( b . next === null ) { this . tail = a . prev ; } else { b . next . prev = a . prev ; } return this ; }
Removes a list of vertices whose 'head' is 'a' and whose 'tail' is b
Removes a list of vertices whose head is a and whose tail is b