How to use [include]

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

This slot is logically the combination of a [for-each], [add] and [eval/invoke]. The slot takes a lambda object, that will be executed with a [.dp] node, for each node resulting from evaluating its expression passed in by reference. The returned nodes will then be appended into the currently iterated node, allowing you to “include” sub trees into a destination tree using a declarative syntax. It is useful for adding nodes dynamically to an existing graph object, declaratively, according to the state of its destination nodes.

.data
   foo1:bar1
   foo2:bar2
   foo3:bar3
include:x:@.data/*
   strings.concat
      get-value:x:@.dp/#
      .:" howdy"
   unwrap:x:+/*
   return
      howdy:x:@strings.concat