return function(key, rule, mandatory)
    if mandatory == nil then
        mandatory = true
    end

    return {
        messages = {
            affirmative = {
                standard = "{{key}} must be defined"
            },
            negative = {
                standard = "{{key}} must not be defined"
            }
        },
        apply = function(context)
            context.key = key

            if context.input[key] == nil then
                context.result = (mandatory == false)
                return
            end

            if not rule then
                context.result = true
                return
            end

            local child_context = context:new_child(rule)
            child_context.name = key
            child_context.input = context.input[key]
            child_context:apply_rule()

            context.result = child_context.result
        end
    }
end