Here is an example of a deep merge, using the knockout_prefix setting to specify a prefix to indicate a value should be removed from the result:

# common.yaml
 ---
 classification:
   classes:
     - paessler
     - other
# mynode.myorg.net.yaml
 classification:
   classes:
     - -- paessler
     - nagios
     - webserver

Here, we are indicating that mynode.myorg.net.yaml is not using Paessler for monitoring, but rather Nagios. The use of this lookup returns the correct value, as follows:

lookup({
 'name' => 'classification',
 'merge' => {
 'strategy' => 'deep',
 'knockout_prefix' => '--',
 },
 })