implement sum properly in docx mathml

starmath/source/ooxmlexport.cxx

@@ -472,6 +472,7 @@ void SmOoxmlExport::HandleOperator( const SmOperNode* pNode, int nLevel )
         case TLLLINT:
         case TPROD:
         case TCOPROD:
+        case TSUM:
         {
             const SmSubSupNode* subsup = pNode->GetSubNode( 0 )->GetType() == NSUBSUP
                 ? static_cast< const SmSubSupNode* >( pNode->GetSubNode( 0 )) : NULL;

starmath/source/ooxmlimport.cxx

@@ -486,6 +486,9 @@ OUString SmOoxmlImport::handleNary()
         case MS_COPROD:
             ret = STR( "coprod" );
             break;
+        case MS_SUM:
+            ret = STR( "sum" );
+            break;
         default:
             fprintf( stderr, "Unknown m:nary chr '%d'\n", chr );
             break;