The .default() function is used internally to mark certain values as
default, so that the user may be notified when default values are being used.
For example, choosing a default reference treatment for a network, or using
default prior distributions. The function .is_default() checks whether an
argument/object is set to a default value. Neither of these functions are
intended to be called by the user.
Usage
.default(x = list())
.is_default(x)