 nabla = backward difference,
                                     U+2207 ISOtech --&gt;
&lt;!ENTITY isin     CDATA "&amp;#8712;" -- element of, U+2208 ISOtech --&gt;
&lt;!ENTITY notin    CDATA "&amp;#8713;" -- not an element of, U+2209 ISOtech --&gt;
&lt;!ENTITY ni       CDATA "&amp;#8715;" -- contains as member, U+220B ISOtech --&gt;
&lt;!-- should there be a more memorable name than 'ni'? --&gt;
&lt;!ENTITY prod     CDATA "&amp;#8719;" -- n-ary product = product sign,
                                    