FIY the Wikipedia article rightfully says SGML CONCUR usage is uncommon, but compared to the stated alternatives for overlapping markup, it's basically the only one that is tolerable to use as actual markup language for use with a text editor. This is what it looks like:

    <!doctype d -- element decls for a, b ... -->
    <!doctype e -- element decls for a, x ... -->
    <(d|e)a>
      <(d)b>bla bla <(e)x>bla </(d)b> bla</(e)x>
    </(d|e)a>

where the third "bla" span is marked up with overlap.

Basically, in case you've ever wondered, SGML CONCUR is the only reason that the element name in end-element tags needs to be specified. In strictly nested markup (XML) it always must refer to the most recently opened start-element tag hence it's redundant. SGML actually has "</>" but it didn't make it into XML.