7222

Ok, I found a failing manual test for files as well.

Here are the two steelman tests, one for screens and one for files:

1.
  5 5 fake-screen =s

  s 1 down 1 right

  ctrl-d foo

  expand

final state:

  s       foo                                            foo
               s       1       down    1       right   ⇗
   ┌─────┐                                                ┌─────┐
                ┌─────┐      1  ┌─────┐      1  ┌─────┐    │
                        ┌─────┐  │      ┌─────┐  │         ─
                                         │       │
                                         │       │
                                                 ─
   └─────┘                                                └─────┘
                └─────┘         └─────┘         └─────┘
                        └─────┘         └─────┘

2.

  "x" open =f

  f read f read

  ctrl-d read2

  expand

final state:

  f      read2                                read2
                f      read   f      read   ⇗
    FILE                                       ❝def❞
                  FILE  ❝abc❞   FILE  ❝❞
                               ❝def❞  ❝ghi❞

In both cases there are 3 levels of issues:

- getting a single-line expression to work
- getting a single-line expression to work when operating on a binding
  defined in a previous line
- getting an expanded function call to work

The third is where the rub is right now. And what both examples above share
is that the function performs 2 mutations to the screen/file.

So we need a deep copy after all. And it's not very clear how to copy a
file descriptor including the seek location. Linux's dup() syscall creates
an alias to the file descriptor. And opening /proc seems awfully Linux-specific:

https://stackoverflow.com/questions/54727231/duplicating-file-descriptor-and-seeking-through-both-of-them-independently/54727424#54727424

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