_ = setmetatable({}, {
    __call = function() end,
    __pow = function(self, def)
        getfenv()[def[1]] = (
            type(def[2]) == "function" and setmetatable({def[2]},{
                __index = function(self2, other2)
                    self2[1](unpack(other2))
                end
            }) or def[2]
        )
    end
})

_(_ ^ {"sum", function(...)
    _(_ ^ {"x", 0})
    for i = 1, #({...}) do
        x = x + ({...})[i]
    end
    print(x)
end})

_(sum[{5, 10, 15}])