Clipper? (A -quirky - DBF format besides being a language too, widely used in the past century)
Clipper indices are difficult to get right.
And, Ahum, PerfectView's DBF format, which was widely used (~ 300.000 licenses) in the same era in The Netherlands? (I can help you with that: I know the index format, which is quirky but nice, the company pays part of my pension)
Seriously, though, world-wide Clipper indexing is a real miss..