Change tee or add a new tee that provides the copy iterator into a new scope

[cjrh]

Perhaps something like:

(
  Iter.open(filename)
  .tee(
      # This is the second iterator, provided to a callable
      lambda it: (
        it
        .map(str.lower)
        .write_to_file("lower.txt")
      )
  )
  # This continues with the original lines iterator
  .map(str.upper)
  .write_to_file("upper.txt")
)

[cjrh]

Need to understand carefully what runs when in examples like these. Should tee internally pass the second iterator into the callback, wait for that to return, and then yield? Not sure there's another way to do it.