“…It is also reasonable to consider settings in which there is a particular value signaling non-halting, which the computable function may never identify. This approach is explored by Crook et al [10], where nonhalting of a learner's output is signaled by the value ⊥. A related approach is considered by Calvert [9], who studies PAC learning for concepts that are Π 0 1 classes on 2 N , which can be thought of as equivalent to working with computable functions from 2 N to Sierpiński space S (i.e., the space {⊥, ⊤} with open sets {∅, {⊤}, {⊥, ⊤}}), where the inverse image of ⊤ is the Π 0 1 class in question.…”