Path Expressions (Predicates)

Predicates

result lists can be filtered
postfix operator on lists [], any number of these (implicit AND)
```
... \ is-author-of / author [ bn = "Shakespear" ]
```
predicates can be of the form path-expression comparison path-expression or of the form path-expression
constants are treated as pathological path expressions
predicate path expression are always evaluated on lists
only those list members 'survive' where the predicate returns something non-empty
it is NOT possible to introduce new variables in a path expression