class ibool
    function __construct(b)
        assert(type(b) == "boolean", "argument must be a boolean")
        self.b = b
    end

    function iff(cond) return cond ? self.b : not self.b end
end

local itrue = new ibool(true)

local need_umbrella
local raining = true

-- Normal syntax.
if raining then need_umbrella = true end
print($"Is it raining? {raining}. Do I need an umbrella? {need_umbrella}")

-- Inverted syntax.
raining = false
need_umbrella = itrue:iff(raining)
print($"Is it raining? {raining}. Do I need an umbrella? {need_umbrella}")