the prefix is one of the following:
<code>fig:</code>,
<code>tbl:</code>,
<code>box:</code>,
<code>chap:</code>,
<code>sec:</code>,
<code>eq:</code>,
<code>subsec:</code>,
<code>subsubsec:</code>,
<code>para:</code>
<code>paragraph:</code>.
Checks also that chapter labels are marked with <code>chap:</code>. 
(N.B. although each label must have a prefix, it must not necessarily the <em>right</em> prefix; 
for example, a table caption may have prefix <code>tbl:</code>.)</p>
    

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