How to use [not-null]

The reference page of not-null has its arguments, constraints and declaration.

[not-null] will evaluate to true if its specified expression returns nodes and at least one of those nodes has a value. If not, it will return false.

.src1
   foo:foo
.src2
   foo
.src3
not-null:x:@.src1/*
not-null:x:@.src2/*
not-null:x:@.src3/*