SWI-Prolog -- default/3

default(+Arg, +Default, -Value)

Handle default arguments in methods. default/3 is only used inside a user-defined method declaration. It specifies the value of an omitted (default) argument. If Arg is not omitted its value is returned in Value; otherwise Value is unified with Default.

Default may take the form

resource(Object, ResourceName)

in which case Value will be bound to the result of object<-resource_value: ResourceName.

Note that the message-passing kernel passes omitted arguments as @default. Example:

image(R, I:[image]) :->
    "Set image of object"::
    default(I, image('pce.bm'), Image),
    ...