For every issue string, json['issues'][issue_string] contains a list
of (page, count_of_issue_on_this_page) tuples.
To get the total number the issue appears in the current document,
we need to add up all the count_of_issue_on_this_page, not multiply the
page number with count_of_issue_on_this_page and add those (－‸ლ)
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