You are in a hole.

A hole is an incomplete expression that was not given a value before running the
program. In the code, a hole can be indicated by any of the following forms:

    ?
    ?name
    ?{Commented out code}
    ?name{Commented out code}

If Quail attempts to evalute a hole, execution pauses and you must fill in a
value before continuing. Once filled, a hole will preserve that value if it is
evaluted again.

To fill a hole, write:

    > fill <expr>

where <expr> is an expression like `zero` or `cons zero nil`.

You may also evaluate expressions without filling the hole with:

    > eval <expr>

If you would like to abort the program instead of filling the hole, use:

    > abort