(when (> *delivery-level* 0) :delete)
The value is one of:
Do not delete any condition class. This is the default at delivery level 0.
Delete unwanted conditions. If an error for a deleted condition is signaled, it is signalled as a simple error condition, with the arguments in the
slot. This is the default at delivery level > 0.
Redirect unwanted conditions to the first parent in their hierachy which is not deleted.
See Deleting of condition classes.