code: function filterElement( element, parentName, isFirst, isLast ) {
  			var nodes = filterElementInner( element, parentName ),
  				nodes2 = [],
  				nodesCount = nodes.length,
  				nodeIndex = 0,
  				node,
  				afterSpace = 0,
  				lastSpaceIndex = -1;
  
  			// Remove duplicated spaces and spaces at the:
  			// * beginnig if filtered element isFirst (isFirst that's going to be inserted)
  			// * end if filtered element isLast.
  			for ( ; nodeIndex < nodesCount; nodeIndex++ ) {
  				node = nodes[ nodeIndex ];
  
  				if ( node == ' ' ) {
  					// Don't push doubled space and if it's leading space for insertion.
  					if ( !afterSpace && !( isFirst && !nodeIndex ) ) {
  						nodes2.push( new CKEDITOR.dom.text( ' ' ) );
  						lastSpaceIndex = nodes2.length;
  					}
  					afterSpace = 1;
  				} else {
  					nodes2.push( node );
  					afterSpace = 0;
  				}
  			}
  
  			// Remove trailing space.
  			if ( isLast && lastSpaceIndex == nodes2.length )
  				nodes2.pop();
  
  			return nodes2;
  		}
docs: TODO: Review content transformation rules on filtering element.