Default value modification of a leaf which is of type identityref

identity xyz {
}
identity x
{
base “xyz”;
}
identity y
{
base “xyz”;
}
identity z
{
base “xyz”;
}

leaf mode {
type identityref {
base “xyz”;
}
default x;
}

In deviation YANG file, I need to change the default value to ‘y’ and also restrict the configuration of other values ‘x’ and ‘z’;

Kindly help how to do this…