Determining whether an Instant occurs in the past is painfully verbose:

Temporal.Instant.compare(instant, Temporal.Now.instant()) < 0

But it seems like the API design is very focused on "only one way to do it", regardless of convenience.

I really want to introduce generic comparison (not operators) at some point, i.e.:

interface Equaler {
  equals(a, b): boolean;
  hashCode(x): number;
}
interface Comparer {
  compareTo(a, b): number;
}

And allow you to pass an Equaler to a Map/Set constructor, and add things like a SortedMap/StortedSet like I put together in @esfx/equaler, @esfx/collections-hashmap, @esfx/collections-hashset, @esfx/collections-stortedmap, @esfx/collections-sortedset (which I wrote as a test bed to test out the implementation).

ok, I think I fixed my breaking changes. @esfx/equatable defines the following:

• Equatable - A "protocol" involving two symbol-named methods ([Equatable.equals](other) and [Equatable.hash]()) indicating an object defines its own equality mechanism.
• Comparable - A "protocol" involving one symbol-named method ([Comparable.compareTo](other)) indicating an object defines its own comparability mechanism.
• StructuralEquatable - A "protocol" for [StructuralEquatable.structuralEquals](other, equaler) and [StructuralEquatable.structuralHash](equaler)
• StructuralComparable - A "protocol" for [StructuralComparable.structuralCompareTo(other, comparer).
• Equaler - An "interface" describing an object with { equals(a, b), hash(x) } methods, as well as a namespace object with default implementations (defaultEqualer, structuralEqualer, tupleEqualer (actually compares arrays), tupleStructuralEqualer), a create helper method to create an Equaler from callbacks, and a combineHashes function to help with complex hashCode construction (via bit shifting and XOR).
• Comparer - An "interface" describing an object with { compare(a, b) } methods, as well as a namespace object with default implementations (defaultComparer, structuralComparer, tupleComparer, and tupleStructuralComparer).

There's also a @esfx/equatable-shim package that shims the Equatable and Comparable interfaces onto built-ins (and all of those methods are symbol-named, so no issues like with .flatten, etc.)