Consider this example:

  proc show_invocation {} {
      return [info level -1]
  }


  proc a args {
      puts "Beginning of a."
      puts [show_invocation]
  }


  proc b args {
      a x y z
  }

  proc c args {
      b 1 2 3
  }

  c skip-to-my-lou

It has output 

  Beginning of a.
  a x y z

In English words, this illustrates that a [proc] can report on the command-line
which invoked it.  In particular, this provides a base for introspection on the
'''proc''' call-stack, coded in "[Print proc sequence]", "[A basic debugger]", 
"[List the call stack]", and perhaps elsewhere.



----

There's more, though:  with 8.5, it becomes possible to write

  package require Tcl 8.5

  proc show_complete_invocation {} {
      array set frame [info frame -2]
          # Memory leak ahoy!  In real code, the file handle deserves its [close].
      set lines [split [read [open $frame(file)]] \n]
      return [lindex $lines [expr $frame(line) -1]]
  }


  proc a args {
      puts "Beginning of a."
      puts [show_complete_invocation]
  }


  proc b args {
      a x y z
      set x [a b c d]
  }

  proc c args {
      b 1 2 3
  }

  c skip-to-my-lou

The output of ''this'' script is

    Beginning of a.
          a x y z
    Beginning of a.
          set x [a b c d]

While this is a bit fragile, in that it requires a few more lines to
handle dynamic '''proc''' definitons, and other complications, it should at least
hint at the possibilities of this sort of introspection.



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