Closure trait for loop entry

[Joe Gottman]

   Often when I write a loop I want to run some code at loop entry time.  It
would be nice to have a closure trait for this, similar to NEXT for loop
continuation or LAST for loop termination, but there isn't one.  I don't
think either FIRST or ENTER do quite what I want.  FIRST runs only once,
which is too few times, and ENTER runs every time the loop block is entered,
which is too many.

   To demonstrate what I want, consider the following examples:

   sub use_first() 
   {
        for 1..2 {
                FIRST {say 'entering loop';}
                say $_; 
                LAST    {say 'leaving loop';}
        }
  }

  sub use_enter()  
   {
        for 1..2 {
                ENTER {say 'entering loop';}
                say $_; 
                LAST    {say 'leaving loop';}
        }
  }


The first time use_first is called it will print
        entering loop
        1
        2
        leaving loop

but subsequently it will print
        1
        2
        leaving loop

and leave out the 'entering loop'.

When use_enter is called it will print
        entering loop
        1
        entering loop
        2
        leaving loop

I want to output
        entering loop
        1
        2
        leaving loop

every time I run my loop. I suggest we create a new closure trait called
SETUP for this.  Then the code could read

        for 1..2 {
                SETUP {say 'entering loop';}
                say $_; 
                LAST    {say 'leaving loop';}
        }

Since SETUP can be used for initialization, it should probably be allowed
within an expression:
        state $first_iteration = SETUP {true} will next {$_ = false};

Lower case 'setup' could probably also be used as a trait on a variable
        state $first_iteration will setup {$_ = true} will next {$_ =
false};

Joe Gottman