gistfile1.txt

Here my first improvements:

    subset([], List2) <= (List2 == List2)
    subset(L1, L2) <= (
        (len_(L1) > 0) &
        (L1[0]._in(L2)) & # first element in L2
        subset(L1[1:], L2) # remainder is a subset of L2
    )
    
I still get DatalogError: Syntax error: Incorrect use of aggregation. And of course there has to be a more elegant way of saying True (List2 == List2).