x'</span>,<span class="num">1</span>,<span class="lit">'a'</span>), (<span class="lit">'x'</span>,<span class="num">1</span>,<span class="lit">'b'</span>), (<span class="lit">'y'</span>,<span class="num">2</span>,<span class="lit">'a'</span>), (<span class="lit">'y'</span>,<span class="num">2</span>,<span class="lit">'b'</span>))</pre></div><dl class="paramcmts block"><dt class="tparam">I</dt><dd class="cmt"><p>Type of the elements of the left index</p></dd><dt class="tparam">J</dt><dd class="cmt"><p>Type o