Function parse_set_relation_pair 

pub(crate) fn parse_set_relation_pair(
    pair: Pair<'_, Rule>,
    input_children: Vec<Rel>,
    child_field_counts: &[usize],
    advanced_extension: Option<AdvancedExtension>,
) -> Result<(Rel, usize), MessageParseError>

Parse a set_relation pair given the real per-child output widths (not just their sum), so mismatched input schemas are always caught