This extends the command string to accept a new sub-command '''concat''':

 if {[catch {string concat}]} then {
    rename string STRING_ORIGINAL
    proc string {cmd args} {
        switch -regexp -- $cmd {
            ^con(c(a(t)?)?)?$ {
                uplevel [list join $args {}]
            }
            default {
                if {[catch {
                    set result\
			[uplevel [list STRING_ORIGINAL $cmd] $args]
                } err]} then {
                    return -code error\
                        [STRING_ORIGINAL map\
                             [list\
                                  STRING_ORIGINAL string\
				  ", compare, equal,"\
				  ", compare, concat, equal,"]\
                             $err]
                } else {
                    set result
                }
            }
        }
    }
 }

Test if it does as expected:

 % string concat hully gully
 hullygully
 %

Yeah. Check an original sub-cmd:

 % string match -nocase hully gully
 0
 % 

Works. Great. Now some erraneous situation:

 % string match -nocase hully gully bully
 wrong # args: should be "string match ?-nocase? pattern string"
 % 

The err msg hides the ''STRING_ORIGINAL'' and shows up ''string'' instead. Great again. Now another situation:

% string what'dya'mean?
bad option "what'dya'mean?": must be bytelength, compare, concat, equal, (...)
% 

The err msg shows up all sub-cmds inclusive concat. Yep. That's it. Errp.