Labels
      module A
      
       : sig ... 
       end
      
     
    
      val f : t
      
     
    Attached to value
      val e : 
       unit -> 
       t
      
     
    Attached to external
      module 
       type 
       S
      
       = sig ... 
       end
      
     
    class 
      c
       : object ... 
       end
      
     
    
      class 
       type  
      cs
       = object ... 
       end
      
     
    Attached to extension
      module S := 
       A
      
     
    Attached to module subst
type s
       := t
     
    Attached to type subst
Testing that labels can be referenced