Function recognize_zip_sum_where 

pub fn recognize_zip_sum_where(
    stmts: &[Stmt<'_>],
    interner: &Interner,
    wanted: &dyn Fn(&ZipSum) -> bool,
) -> Result<ZipSum, ZipReject>

recognize_zip_sum for the first loop whose zip sum wanted accepts; a zip sum it turns away counts as ZipReject::UnsupportedTerm (its term is not the one asked for).