Feature request: a builtin predicate for "get the first n elements of a list," where n is given as a subscript. The closest I can think of is a₀l₃ (with *n*=3 in this case)--and that construct gets more complicated if you want to use a subscript of ₍ or ₎.
It might be possible to accomplish this with a subscript on h (and similarly for t with the last n elements), except I'm not sure how that would work with the non-subscripted case, which outputs an element, not a list containing a single element. Is it possible for [1,2,3]h to output 1 while [1,2,3]h₁ outputs [1] (and [1,2,3]h₀ outputs [] for completeness)?
@DLosc This is now implemented as of this commit. As you can see, I removed a TODO comment about that very feature, which dates back to the first commit of Brachylog v2 on January 22, 2017. Better late than never :p