A module internal to Incremental. Users should see Incremental_intf.

An `At` is a kind of DAG node whose value is `Before` until the clock reaches a certain time, at which point its value becomes `After`.

`include module type of sig ... end`
`type t``= Incremental_kernel__Types.At.t``=` `{`

`main : Incremental_kernel__.Before_or_after.t Incremental_kernel__Types.Node.t;`
`at : Incremental_kernel__.Import.Time_ns.t;`
`mutable alarm : Incremental_kernel__Types.Alarm.t;`

`}```
`include Core_kernel.Invariant.S with type t := t`
`type t`````
`val invariant : t Base__.Invariant_intf.inv`
`include Incremental_kernel__.Sexp_of.S with type t := t`
`type t`````
`include sig ... end`
`val sexp_of_t : t ‑> Base.Sexp.t`