In [KitSubst], implement [visit_'fn] as the identity,

so that substitution succeeds even if there are names
of type ['fn] which the substitution does not rewrite.

src/KitSubst.ml

@@ -19,10 +19,14 @@ let extend x env =
 
 class ['self] map = object (_ : 'self)
   method private extend = extend
-  (* [visit_'fn] is not implemented, as it is up to the user to identify
-     variable nodes and replace them. This requires overriding a suitable
-     visitor method. *)
-  method private visit_'fn _env _x = assert false
+  (* [visit_'fn] must be implemented. There could be several kinds of
+     nodes that carry variables of type ['fn], say, [FooVar] and [BarVar],
+     and the user may decide to substitute away [FooVar] nodes, but leave
+     [BarVar] nodes untouched. Another reason is, in an encoding of ML
+     disjunction patterns where a bound name can appear several times,
+     things may be set up so that [visit_'bn] is called at the first
+     occurrence and [visit_'fn] is called at subsequent occurrences. *)
+  method private visit_'fn _env x = x
 end
 
 let apply (copy : 'thing -> 'thing)