Find property-based test ideas

Code with a large input space where example tests only scratch it.

Suggest property-based tests for the code below.

Look for properties of these kinds:
- Invariants that hold for all inputs: length, ordering, sign, membership, conservation of total.
- Round trips: encode then decode, serialise then parse, do then undo.
- Equivalence: this implementation agrees with a slower obvious one.
- Idempotence: applying twice equals applying once.
- Commutativity or associativity, where the operation claims it.
- Metamorphic relations: how the output must change when the input changes in a known way.

For each property: state it in plain language, then as test code, with a suitable generator. Note which inputs the generator must be able to produce for the property to be meaningful, and flag any property you suspect is false for this code.

Code:

Finding coverage gaps. Working out which behaviours, edge cases and error paths nothing currently checks.