You can use proc command in Tcl to replace existing commands and procedures, which gives you the possibility to implement different behaviour.

An example that shows this:

  ~$ tclsh
  % rename while origwhile
  % proc while {args} {
      puts stderr "Debug: invoked while with arguments $args"
      # do anything you want here
      uplevel 1 origwhile $args
    }
  % set i 0
  % while {$i < 5} {puts $i; incr i}
  Debug: invoked while with arguments {$i < 5} {puts $i; incr i}
  0
  1
  2
  3
  4
  % 

And in Forth the dictionary is searched backwards for a word (starting with the most recent definitions), so you can similarly override words. Powerful!