message: Add stratification rules for annotations
diff: context : type.
  + %extend context.
  +
  + eval : context -> ast.stratified.kind -> ast.stratified.semantics.kind_value -> prop.
  + % TODO: eval
  +
  + eval : context -> ast.stratified.typ -> ast.stratified.semantics.typ_value -> prop.
  + % TODO: eval
  +
  + eval : context -> ast.stratified.expr -> ast.stratified.semantics.expr_value -> prop.
  + % TODO: eval
  +
  + %end.
  +
  % Check that a term is a type and lower it to a stratified term.
  is_type : context -> ast.core.term -> stratified_term -> prop.
  % Conversion
  % Variables
  +
  % Annotated terms
  + is_type Context (ast.core.ann Elem Type) Elem' :-
  + is_type Context Type (kind Kind),
  + context.eval Context Kind Kind',
  + check_type Context Elem (kind Kind') Elem'.
  + is_type Context (ast.core.ann Elem Type) Elem' :-
  + is_type Context Type (typ Type'),
  + context.eval Context Type' Type'',
  + check_type Context Elem (typ Type'') Elem'.
  % Universes
  is_type Context ast.core.type_type (kind ast.stratified.type_type).
  