18.4.5.2 Motion Protocol

The concept of a basic motion function was introduced in Unit Protocol. A general motion function is a function named forward-unit or backward-unit of the signature (mark syntax &optional (count 1) (limit-action #'ERROR-LIMIT-ACTION)) that returns true if it could move forward or backward over the requested number of units, count, which may be positive or negative; and calls the limit action if it could not, or returns NIL if the limit action is NIL.