faster (make-prompt-tag); default-prompt-tag is a parameter

* module/ice-9/boot-9.scm (default-prompt-tag): Once parameters have
  booted, redefine as a parameter.
  (make-prompt-tag): Change from a gensym to a list.  Thanks to Mark
  Weaver for the suggestion.

* doc/ref/api-control.texi (Prompt Primitives): Update docs.

doc/ref/api-control.texi

@@ -494,14 +494,17 @@ those passed to @code{abort-to-prompt}.
 @end deffn
 
 @deffn {Scheme Procedure} make-prompt-tag [stem]
-Make a new prompt tag.  Currently prompt tags are generated symbols.
+Make a new prompt tag.  A prompt tag is simply a unique object.
-This may change in some future Guile version.
+Currently, a prompt tag is a fresh pair.  This may change in some future
+Guile version.
 @end deffn
 
 @deffn {Scheme Procedure} default-prompt-tag
 Return the default prompt tag.  Having a distinguished default prompt
 tag allows some useful prompt and abort idioms, discussed in the next
-section.
+section.  Note that @code{default-prompt-tag} is actually a parameter,
+and so may be dynamically rebound using @code{parameterize}.
+@xref{Parameters}.
 @end deffn
 
 @deffn {Scheme Procedure} abort-to-prompt tag val1 val2 @dots{}

module/ice-9/boot-9.scm

@@ -51,10 +51,12 @@
 
 (define make-prompt-tag
   (lambda* (#:optional (stem "prompt"))
-    (gensym stem)))
+    ;; The only property that prompt tags need have is uniqueness in the
+    ;; sense of eq?.  A one-element list will serve nicely.
+    (list stem)))
 
 (define default-prompt-tag
-  ;; not sure if we should expose this to the user as a fluid
+  ;; Redefined later to be a parameter.
   (let ((%default-prompt-tag (make-prompt-tag)))
     (lambda ()
       %default-prompt-tag)))
@@ -1326,6 +1328,14 @@ VALUE."
 
 
 
+;;; Once parameters have booted, define the default prompt tag as being
+;;; a parameter.
+;;;
+
+(set! default-prompt-tag (make-parameter (default-prompt-tag)))
+
+
+
 ;;; Current ports as parameters.
 ;;;