nl_tokens: Removes a list of vertices whose head is a and whose tail is b
pl_tokens: function ( a , b ) { if ( a . prev === null ) { this . <mask> = b . next ; } else { a . prev . next = b . next ; } if ( b . next === null ) { this . tail = a . prev ; } else { b . next . prev = a . prev ; } return this ; }