Identify outermost brace indices in list of indices.
The purpose here is to find the brace pairs that correspond to BibTeX entries,
while discarding brace pairs that appear within the entries
(e.g., braces around article titles).
Parameters:
idx_list (list of tuple of int) – A list of two-element tuples of indices of matched braces.
Returns:
reduced_idx_list (list of tuple of int) – A list of two-element tuples of indices of matched braces corresponding to BibTeX entries.