public function focusIndex( $column, $value ){
		if( !in_array( $column, $this->indices ) && $column != $this->primaryKey )				//  check column name
			throw new \InvalidArgumentException( 'Column "'.$column.'" is neither an index nor primary key and cannot be focused' );
		$this->focusedIndices[$column] = $value;													//  set Focus
	}