code: @Override
  	public SqlFilterManager addSqlFilter(final SqlFilter filter) {
  		if (filter == null) {
  			throw new IllegalArgumentException("filter is null.");
  		}
  		filters.add(filter);
  		return this;
  	}
docs: {@inheritDoc}
  
  @see jp.co.future.uroborosql.filter.SqlFilterManager#addSqlFilter(jp.co.future.uroborosql.filter.SqlFilter)