Symboolsommen

Picture equations (symboolsommen): sums in which pictures stand for numbers,
like apple + apple = 10 and apple + banana = 8. The pupil finds what each
picture is worth: reasoning with unknowns, a first step towards algebra. The
booklet shows the sums in a frame; on the answer sheet the pupil writes the
value next to each picture, and optionally the answer to a last sum
(`question`). Graded per box.

| Setting | What it does |
| --- | --- |
| `symbols` | 1 to 4 pictures: `id` (a short name in the bundle's language, used in the sums, on the answer sheet and in the worked solution), `image` (from the library, `@emoji/red-apple`) and the hidden `value`. |
| `equations` | 1 to 6 sums: `terms` (symbol ids and plain numbers, added) and `result`. |
| `ask` | The pictures the pupil writes a value for. Default: all. |
| `question` | A last sum with the pictures, worked out with the values found. |

Validation checks every sum with the values, that each picture is in a sum,
and that the sums fit only one set of values.

Add a `solution`: one move per picture. `equation` (one sum in which it is the
only unknown picture: `equations: [2]`), `compare` (two sums where every
picture of the second is also in the first: the difference has only this
picture, `equations: [1, 2]`) or `try`. The manual shows the steps and the
booklet gets a hints sheet.

Easier: a sum with only one kind of picture first. Harder: no such sum, so
the pupil has to compare two sums; three or four pictures; plain numbers in
the sums.

```yaml
- type: task
  id: q1
  prompt: Wat is elk stuk fruit waard?
  answer:
    type: tempono/picture-equations@1.0.0
    symbols:
    - { id: appel, image: "@emoji/red-apple", value: 5 }
    - { id: banaan, image: "@emoji/banana", value: 3 }
    equations:
    - { terms: [ appel, appel ], result: 10 }
    - { terms: [ appel, banaan ], result: 8 }
    solution:
    - { symbol: appel, rule: equation, equations: [ 1 ], value: 5 }
    - { symbol: banaan, rule: equation, equations: [ 2 ], value: 3 }
```