TomoStash/_includes/tome-parent

{%- capture tome-id -%}
    {%- include tome-id tome=include.tome -%}
{%- endcapture -%}
{%- assign id-parts = tome-id | split: '/' -%}
{%- assign key = include.key | strip -%}
{%- if id-parts.size == 2 and id-parts[1] != 'index' -%}
    {%- if key != '' -%}
        {%- for tome in site.tomes -%}
            {%- capture parent-id -%}
                {%- include tome-id tome=tome -%}
            {%- endcapture -%}
            {%- assign parents-parts = parent-id | strip | tome-id | split: '/' -%}
            {%- if parents-parts[1] == 'index' and parents-parts[0] == id-parts[0] -%}
                {{- tome[include.key] -}}
            {%- endif -%}
        {%- endfor -%}
    {%- else -%}
        {{- id-parts[0] -}}
    {%- endif -%}
{%- endif -%}