let counter = setmeta({
    count = 0
}, {
    __type = "counter"
    __tostring = fn (self) {
        return "%s(%s)":format(type(self), self.count)
    }
    __call = fn (self) {
        raw_set(self, "count", self.count + 1)
        return self.count
    }
    __get = fn (self, key) {
        return raw_get(self, key)
    }
    __set = fn (self, key, value) {
        if key == "count" {
            error "can not change counter manually"
        } else {
            raw_set(self, key, value)
        }
    }
})
print(type(counter)) # counter

print(counter) # counter(0)
print(counter()) # 1
print(counter) # counter(1)
print(counter()) # 2
print(counter) # counter(2)
print(counter()) # 3
print(counter) # counter(3)

print("counter.count =", counter.count) # counter.count = 3
counter.a = false
print("counter.a =", counter.a) # counter.a = false
counter.count += 1 # ERROR: can not change counter manually