To return a string, the ExpressionCompiler requires a quoted string `"http://url"`. So if we get rid of the prefix, we'd have to do something like for everything that is not an expression that in the yaml:

```yaml
relations:
    -
        rel: "'friends'"
```

So I figured, let's add a prefix for expressions. Maybe we could change it from eval to something else. Or if you think of a better approach let me know