Statement.OpaqueType
type ('M, 'T) t = {
id : ('M, 'T) Identifier.t;
tparams : ('M, 'T) Type.TypeParams.t option;
impltype : ('M, 'T) Type.t option;
supertype : ('M, 'T) Type.t option;
comments : ('M, unit) Syntax.t option;
}