next: Disjunctions of Structs

## Default Values

Elements of a disjunction may be marked as preferred. If there is only one mark, or the users constraints a field enough such that only one mark remains, that value is the default value.

In the example, replicas defaults to 1. In the case of protocol, however, there are multiple definitions with different, mutually incompatible defaults. In that case, both "tcp" and "udp" are preferred and one must explicitly specify either "tcp" or "udp" as if no marks were given.

defaults.cue

// any positive number, 1 is the default
replicas: uint | *1

// the default value is ambiguous
protocol: *"tcp" | "udp"
protocol: *"udp" | "tcp"

\$ cue eval defaults.cue

replicas: 1
protocol: "tcp" | "udp" | *_|_