Rotate a list. (Creates and returns a new list.)

    proc lrotate {xs {n 1}} {
      if {!$n} {return $xs}
      if {$n<0} {return [lrotate $xs [expr {[llength $xs]+$n}]]}
      
      foreach x [lassign $xs y] {
        lappend ys $x
      }
      lappend ys $y
      lrotate $ys [incr n -1]
    }

By default, rotates a list ''left'' by one element (moving the head to the tail).

Examples:

    % lrotate {1 2 3 4 5}
    2 3 4 5 1

    % lrotate {1 2 3 4 5} -1
    5 1 2 3 4

    % lrotate {1 2 3 4 5} 2
    3 4 5 1 2

    % lrotate {1 2 3 4 5} 0
    1 2 3 4 5

I'm sure someone can come up with a better implementation...