The idea is to provide a limited form of continuations into
a structured programming language, and to generalize generators.

    result = getitem(obj, index, key_error)
    print("ordinary return")
    from key_error(key, value)
        print("other_stuff")


When the program would face a key error, it would return through
the construct with a function call to the key_error -pseudofunction.

The system would also allow continuation to be grabbed, to form a
deep-structured "generator" on-demand.

    depth_first_search(visit)
    from visit(node) cont
        print(node)
        cont(visit)