`type 'a t`````

A `t` represents the edge from a child incremental to a parent expert node. A `t` is stateful, you cannot use the same `t` to link one child node to multiple parents at the same time.

`include sig ... end`
`val sexp_of_t : ('a ‑> Base.Sexp.t) ‑> 'a t ‑> Base.Sexp.t`
`val create : ?⁠on_change:('a ‑> unit) ‑> 'a incremental ‑> 'a t`

When calling `create ?on_change child`, nothing happens until the `t` is linked to a parent. see `Node.add_dependency` for documentation of `on_change`.

`val value : 'a t ‑> 'a`

`value t` reads the value of the child incremental. It can only be used from the callback of the `Expert.Node.t` that has `t` in its set of dependencies.