Concept exercises

This issue is to gather suggestions for concept exercises to form a basic syllabus. The list here will be updated to form the canonical list, but individual concept exercises will have their own issues to track progress.

- [ ] arithmetic
- [ ] primitive reduction over vectors
- [ ] primitive reduction over general arrays (requires: shape, reshape)
- [ ] reduction with user-defined functions
- [ ] dfns (monadic / dyadic / ambivalent / guards)
- [ ] errors (error guards, :Trap, ⎕TRAP maybe)
- [ ] compress (filter using replicate with Boolean mask)
- [ ] selection
- [ ] square bracket index (requires or introduces matrices?)
- [ ] squad
- [ ] pick
- [ ] each (AKA map)
- [ ] stranding of numbers (e.g. `2 2+2 2+2`)
- [ ] parsing simple expressions (long right scope, short left scope)
- [ ] use of parentheses in expressions
- [ ] assignment (single name and pass-through e..g (`var←3` and `var×6+var←12`)
- [ ] reshape
- [ ] stencil (requires: operators?)

Start by reviewing the checklist in this issue and the repository's existing exercise structure. The issue says this list should become the canonical syllabus while individual concept exercises are tracked separately. Done means the concept list has been agreed on and updated, with separate issues available for each exercise.

Written by the indexing model from the issue text.