var s,t: Type;

class b : { x . x < s} %% assume s < t

class a : { x . x < t}

class a < b

class a : {y . y < t } %% ok (but only for simple tokens)

class a : {y . y < s }

class c,d

class a : (c,d)

class e : a %% ok

class h < e %% ok

class h < (a,b) %% ok

class e : { x . x < t }