You can specify keyword-argument pairs in a customization type after the type name symbol. Here are the keywords you can use, and their meanings:
nil is not a valid value for the alternative, then it is
essential to specify a valid default with
If you use this for a type that appears as an alternative inside of
choice; it specifies the default value to use, at first, if and
when the user selects this alternative with the menu in the
Of course, if the actual value of the option fits this alternative, it
will appear showing the actual value, not default.
:actionattribute specifies what the button will do if the user invokes it; its value is a function which takes two arguments—the widget which the button appears in, and the event.
There is no way to specify two different buttons with different
:format, and use ‘%d’ or ‘%h’ in that value.
The usual reason to specify a documentation string for a type is to
provide more information about the meanings of alternatives inside a
:choice type or the parts of some other composite type.
widget-backward, it will display the string motion-doc in the echo area. In addition, motion-doc is used as the mouse
help-echostring and may actually be a function or form evaluated to yield a help string. If it is a function, it is called with one argument, the widget.
nilif the value is acceptable.
nilif the widget's current value is valid for the widget. Otherwise, it should return the widget containing the invalid data, and set that widget's
:errorproperty to a string explaining the error.