OPAL Filter Verbs

Filter verbs select particular rows from the input row set, emitting the matches as output.

Verb	Description
always	When the verb input is a Resource dataset, always selects all resources that matched the predicate at all times.
ever	When the verb input is a Resource dataset, ever selects all resources that at some point matched the predicate.
filter	Exclude rows from the input dataset that do not match the given predicate expression.
never	When the verb input is a Resource dataset, never selects all resources that at no point matched the predicate.
topk	Selects all data for each of top k ranked groups.