code: function mergeCompositionAreasWithPageAreas( composition, definition, containerItems, compositionItem ) {
        forEachArea( composition, (items, areaName) => {
           if( areaName === '.' ) {
              insertAfterEntry( containerItems, compositionItem, items );
              return;
           }
  
           if( !( areaName in definition.areas ) ) {
              definition.areas[ areaName ] = items;
              return;
           }
  
           mergeItemLists( definition.areas[ areaName ], items, definition );
        } );
  
        removeEntry( containerItems, compositionItem );
  
        function insertAfterEntry( arr, entry, replacements ) {
           const index = arr.indexOf( entry );
           arr.splice( index, 0, ...replacements );
        }
  
        function removeEntry( arr, entry ) {
           const index = arr.indexOf( entry );
           arr.splice( index, 1 );
        }
     }
docs: /////////////////////////////////////////////////////////////////////////////////////////////////////////