Module type Splay_tree.Splay_tree0.Reduction_operation

type key
type data
type accum
val identity : accum
val singleton : key:key ‑> data:data ‑> accum
val combine : accum ‑> accum ‑> accum

combine is required to be associative and have identity as its identity. In other words, they must form a monoid.