Function form_dom_driven_repeat_instance_replication 

pub fn form_dom_driven_repeat_instance_replication(
    som: &str,
    form_dom_instance_count: usize,
    template_default_count: usize,
) -> FormDomReplicationDecision

Apply rule form_dom_driven_repeat_instance_replication.

Inputs:

• som — SOM-style name / SOM path of the repeating subform group.
• form_dom_instance_count — number of <subform name="X"> siblings recorded in the saved form DOM for this name.
• template_default_count — number of instances the template’s initial expansion produced (data-driven count after <occur> clamping).

Output:

• clones_to_add = max(0, form_dom_count - template_default_count) when template_default_count > 0. Returns 0 when the template produced no instance at all (the caller’s existing guard) or when the form DOM does not exceed the default count.

Trace anchor

(occur, subform_materialised_from_data) fires when the rule adds at least one clone, with count set to the final instance count (form_dom_instance_count). This reuses the existing M1.5 Reason tag — the rule reports each “instance materialised from form-DOM data” outcome under the canonical XFA Reason vocabulary. Silence on zero-clones is intentional: the rule had no value-add over the template default.

Behaviour preservation

Bit-identical to the inline expression in pdf_xfa::flatten::apply_form_dom_presence:

if xml_count > existing_count && existing_count > 0 {
    let clones_needed = xml_count - existing_count;
    // … clone subtree clones_needed times …
    trace_sites::occur(gname, SubformMaterialisedFromData, xml_count);
}

The rule wraps the if/clones_needed/trace block into a typed decision. The caller still owns the actual clone_subtree / form_children.insert(…) mutations.