unison/unison-src/transcripts/type-modifier-are-optional.md

Type modifiers are optional, unique is the default.

scratch/main> builtins.merge

Types and abilities may be prefixed with either unique or structural. When left unspecified, unique is assumed.

type Abc = Abc
unique type Def = Def
structural type Ghi = Ghi

ability MyAbility where const : a
unique ability MyAbilityU where const : a
structural ability MyAbilityS where const : a