Filter gets all the documents for which the given predicate is true.

Filter can be called on a sequence, selection, or a field containing an array of elements. The return type is the same as the type on which the function was called on. The body of every filter is wrapped in an implicit `.default(false)`, and the default value can be changed by passing the optional argument `default`. Setting this optional argument to `r.error()` will cause any non-existence errors to abort the filter.