Function recognise 

pub fn recognise(
    f: &Core,
    defs: &BTreeMap<Arc<str>, Def>,
    captured: &BTreeSet<VarId>,
) -> Result<Recognised, Refusal>

Try to read a loop’s per-element function as a join.

f is the function as written — a Lam of one parameter, or a global that resolves to one. captured is the set of variables the enclosing plan has operators for, which is what decides whether the collection being looked up in is something the plan can index.