julia> [1,2,3] < [4,5,6]
true
julia> [1,2,3] < [0,5,6]
false
The lt=< suggestion I made applied to scalars. For vectors, it appears that < defers to isless which uses a lexicographic order, which recursively appears to use isless on the elements. Thus, for sortslices, < is no different than the default isless. You’ll need to write your own function that determines whether a vector is less than another vector (using < for the element comparisons rather than isless), then pass that function as lt to sortslices.