Rekenmuur

A number wall (rekenmuur, number pyramid): each brick is the sum of the two bricks directly
below it. Write the **full solution**, top row first. Bricks the pupil must
fill in are `{ ask: n }`; plain numbers are printed. The booklet shows the
wall with empty open bricks; on the answer sheet the pupil writes the number
in the open brick itself, with no digit boxes that would give away how many
digits it has. Each open brick is graded on its own.

Validation checks the sum rule on every brick, and that every open brick
follows from the printed numbers (so there is exactly one answer).

Good puzzles go beyond filling in from the bottom: give the top and some
bricks lower down, so the pupil has to reason backwards (subtract), or give
only the top and the outer bottom bricks, so the pupil has to try and adjust.

Add a `solution`: the moves a pupil makes, in order. Each move works out one
brick: `sum` (both bricks below it are known), `difference` (a brick above it
and the other brick under that one are known: 27 − 12 = 15) or `try` (guess
and check, for walls where neither works yet). Validation replays it, so a
move that needs a brick that isn't known yet is an error on that move. The
teacher manual then shows the solution step by step, the teacher view plays
it on the wall one brick at a time, and the booklet gets a
hints sheet at the back: a hint for the first step and for each point where
adding no longer works.