ἐν πέτροισι πέτρον ἐκτρίβων → by grinding stone against stones
P. and V. λάθρα, λαθραίως (rare P.), P. κρύφα, Ar. and P. κρύβδην, V. κρυφῆ; (also Xen.), κρυφαίως, κρύβδα.
