[Manichean]

For the custom column in my library, I'm seeing the following entry in the OPF:

        <meta name="calibre:user_metadata:#fach" content="{&quot;is_category&quot;: false, &quot;colnum&quot;: 2, &quot;kind&quot;: &quot;field&quot;, &quot;is_custom&quot;: true, &quot;name&quot;: &quot;Fachbuch&quot;, &quot;#extra#&quot;: null, &quot;datatype&quot;: &quot;bool&quot;, &quot;rec_index&quot;: 20, &quot;search_terms&quot;: [&quot;#fach&quot;], &quot;#value#&quot;: true, &quot;link_column&quot;: &quot;value&quot;, &quot;label&quot;: &quot;fach&quot;, &quot;is_editable&quot;: true, &quot;column&quot;: &quot;value&quot;, &quot;is_multiple&quot;: null, &quot;category_sort&quot;: &quot;value&quot;, &quot;table&quot;: &quot;custom_column_2&quot;, &quot;display&quot;: {}}"/>

This is a tristate binary column. From that, I'm guessing that user defined columns are stored in metadata entries of the form

<meta name="calibre:user_metadata:#name_here" content="settings_here"/>

I'm also seeing that extracting the settings would take quite a bit of matching.