2016-05-12: with: use goops to allow overriding enter and exit.

with: use goops to allow overriding enter and exit.

diff --git a/examples/with.w b/examples/with.w
--- a/examples/with.w
+++ b/examples/with.w
@@ -3,17 +3,23 @@
 exec guile -L $(dirname $(dirname $(realpath "$0"))) --language=wisp -e '(@@ (examples with) main)' -s "$0" "$@"
 ; !#
 
+;; A cleaner way to implement this might be using dynamic-wind.
+
 define-module : examples with
 
+import : oop goops
+
 ; import : ice-9 rdelim
 ; with (open-file "with.w") as port
 ;      display : read-line port
 
-define* : enter thing
-        . thing
+define : enter thing
+       . thing
+define-generic enter
 
-define* : exit thing
-        . thing
+define : exit thing
+       . thing
+define-generic exit
 
 define-syntax with
     syntax-rules : as
@@ -24,6 +30,9 @@ define-syntax with
            exit thing
            . res
 
+define-method : exit (thing <port>)
+                close-port thing
+
 define : main args
          with (open-file "with.w" "r") as port
               format #t "~a\n" : read port