/* This skiplist implementation is almost a C translation of the original
* algorithm described by William Pugh in "Skip Lists: A Probabilistic
* Alternative to Balanced Trees", modified in three ways:
- * a) this implementation allows for repeated values.
+ * a) this implementation allows for repeated scores.
* b) the comparison is not just by key (our 'score') but by satellite data.
* c) there is a back pointer, so it's a doubly linked list with the back
* pointers being only at "level 1". This allows to traverse the list