Search for a key in haystack matching predicate pred starting at index in steps of triangular numbers, 0,1,3,6,10,15,21, ... .
Search for a key in haystack matching hit predicate hitPred and hole/tombstone predicate holePred starting at index in steps of triangular numbers, 0,1,3,6,10,15,21, ... .
Probing algorithms used by for instance hash tables.