How to use [validators.default]

The reference page of validators.default has its arguments, constraints and declaration.

This validator takes one or more arguments with any name and any value, and if this node does not exist as a child of the node collection specified as its expression value, it appends the node to it. If the node exists but has a null value, it sets its value to the value of the argument.

.arguments
validators.default:x:@.arguments
   foo1:bar1