repeat introduces a local variable that can collide with code

For example,

    let i 0
    let foo 0
    repeat 20 [ set i i + 1 set foo foo + 1 ]
    output-print (list i foo)

will output `[20 10]` since the loop is compiled to:

    for (var i=0; i < 20; i++) {
      i = i + 1
      foo = foo + 1
    }