Investigate using `deal` annotations

[SamWilsn]

https://deal.readthedocs.io/basic/verification.html

deal is a library for contract-based development, and it comes with limited formal verification and fuzzing support.

Is this something we could use?

[SamWilsn]

Examples

@deal.pre(lambda *args: all(arg > 0 for arg in args))
def sum_positive(*args):
    return sum(args)

sum_positive(1, 2, 3, 4)
# 10

sum_positive(1, 2, -3, 4)
# PreContractError: expected all(arg > 0 for arg in args) (where args=(1, 2, -3, 4))

@deal.post(lambda x: x > 0)
def always_positive_sum(*args):
    return sum(args)

always_positive_sum(2, -3, 4)
# 3

always_positive_sum(2, -3, -4)
# PostContractError: