I'm trying to build a function in SML using `foldr` or `foldl` that will return the logic or and logic and of all elements in a list.

I've tried in this way, using **and** and **or**:

```
fun band x = foldr (op and) true x;
fun bor x = foldr (op or) false x;

```

And also using **andalso** and **orelse**. But I keep receiving error messages such as:

```
Error: unbound variable or constructor: or

```