Smart Key - Argument Completion

When prompting for a Hyperbole argument, a press in the minibuffer:
  ACTION KEY
     Accepts the current minibuffer argument.
  ASSIST KEY
     Offers completions for the current minibuffer argument.

When reading a Hyperbole menu item or an argument with completion:
  ACTION KEY
     Returns the value selected at point if any, else nil.  If the
     value is the same as the contents of the minibuffer, this value is
     accepted as the argument for which the minibuffer is presently
     prompting; otherwise, the minibuffer is erased and the value is
     inserted there, for inspection by the user.
  ASSIST KEY
     Displays Hyperbole menu item help when an item is selected.