This clever idea of Bob Bernecky's  provides a performance improvement for certain expressions by marking certain arrays with special properties. For example, a permutation vector  has the property that it is invariant under various APL primitives such as rotate/reversal (L⌽R and ⌽R) and grade up/down (⍋R and ⍒R).
Bernecky has defined several array predicate properties, one of which has been implemented in NARS so far.
In this case, index generator (⍳R) produces a Permutation Vector, as does deal (L?R) when the left and right arguments are the same — the results of these primitives are marked internally as Permutation Vectors. Further use of such arrays maintains that property when operated on by rotate/reversal and grade up/down. Moreover, both the grade and index of (L⍳R) primitives use a much faster (linear) algorithm than they would normally use.