A liar who always lies says "All my hats are green."(theguardian.com)

    Sum([a, b, c]) = a + b + c -- basically our definition

    Sum([a, b, c]) = Sum([a, b]) + Sum([c]) -- distributive

    Sum([]) = 0 -- identity

    [0] Agg([a, b, c]) = a @ b @ c == ((a @ b) @ c) == (a @ (b @ c)) -- definition + associativity

    [1] Agg([a, b, c]) = Agg([a, b]) @ Agg([c]) -- distributive

    [2] Agg([]) = Identity(@)

  std::all_of(hats.begin(), hats.end(), [](std::string hat) { return hat == "green"; })

  const allHatsGreen = hats.every(hat => hat.color === 'green');

  const matchAllConditions = conditions.every(condition => condition.matches(item))

  let rec all_hats_green = function
    | [] -> true
    | hat::rest -> hat.color = "green" && all_hats_green rest

    let agg = true
    for (const element of mylist) {
      agg = agg || f(element)
    }

    things.every(thing => thing.type != 'hat' || thing.color == 'green')

    things.some(thing => thing.type == 'hat' && thing.color != 'green')