pub fn or(self, other: Variable<T, E>) -> Self {
        Self::Fallback(Box::new(self), Box::new(other))
    }
Combine this Variable with a fallback Variable. Resolving the variable's value will be
done lazily, stopping on the first Variable that successfuly resolves.

# Example Usage

```rust
# use rusoto_credential::Variable;
let primary: Variable<String> = Variable::from_env_var("AWS_SECRET_ACCESS_KEY");
let fallback = Variable::from_env_var("AWS_SECRET_KEY");
let aws_secret_access_key = primary.or(fallback);
```
rusoto/credential/src/variable.rs