Specified in http://www.tcl.tk/cgi-bin/tct/tip/429.html%|%TIP #429%|%.

A pure-Tcl proof of concept using [ensemble extend]:
======
extend string {
    proc cat args {
        join $args ""
    }
}
======

'string cat' acts as the identity function for a single argument (see also: [K]).  For multiple arguments, it concatenates them with no further substitution.