:   '''[namespace] unknown''' ?''script''?

A new feature in Tcl 8.5 that extends the [unknown] mechanism to be customisable
on a per-[namespace] basis. See TIP 181 for details [http://www.tcl.tk/cgi-bin/tct/tip/181].
<<discussion>>
[NEM] ''13 Nov 2006'': Here is an updated version of [let unknown know] that makes use
of 8.5 features: ''namespace unknown'' and [apply]. While (much) longer than the original version,
each "known" snippet can have its own parameters, with all the usual forms accepted by [proc]
(i.e., defaults, [args]). Wrapping of new functionality is essentially completely transparent,
and each new known thing executes as an independent proc, without adding new command names.
You can delegate to the next unknown handler by calling ''next'' (or returning a [continue] 
exception). Without arguments, ''next'' will invoke the next handler with the original command 
line, otherwise it will pass the rewritten form.

 proc know {params body} {
    set ns [uplevel 1 { namespace current }]
    set old [namespace eval $ns { namespace unknown }]
    set new [list $params $body $ns]
    namespace eval $ns [list namespace unknown [list ::known $new $old]]
 }
 proc known {handler old args} {
    set rc [catch { uplevel 1 [linsert $args 0 ::apply $handler] } result]
    if {$rc == 4} {
        # continue - invoke next handler
        if {[llength $result] == 0} {
            uplevel 1 $old $args
        } else {
            uplevel 1 $old $result
        }
    } else {
        return -code $rc $result
    }
 }
 proc next args { return -code continue -level 2 $args }

Examples of use:
 # expand leading word
 know {cmd args} {
   if {[llength $cmd] > 1} { uplevel 1 $cmd $args } else { next }
 }
 # numeric ranges
 proc range {from to} {
     set ret [list]
     while {$from <= $to} { lappend ret $from; incr from }
     return $ret
 }
 know {args} { 
   if {[regexp {^([0-9]+)\.\.([0-9]+)$} $args -> from to]} {
     range $from $to
   } else { next }
 }

[rwm] ''12 Mar 2010'' The above code will not correctly return error codes from the 'known' function.  (IMHO: I note that the 8.5 catch/error/return and the new unknown are not well explained especially return -options) I made the following changes to correct it:

 old --> set rc [catch { uplevel 1 [linsert $args 0 ::apply $handler] } result]
 new --> set rc [catch { uplevel 1 [linsert $args 0 ::apply $handler] } result opts]
 ...
 old --> return -code $rc $result
 new --> return -options $opts $result


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