A symbol to filter. How it is used is handled by the query function.
See Source File
MPL-2
Copyright (c) 2017, Joakim Brännström. All rights reserved.
Joakim Brännström (joakim.brannstrom@gmx.com)
This Source Code Form is subject to the terms of the Mozilla Public License, v.2.0. If a copy of the MPL was not distributed with this file, You can obtain one at http://mozilla.org/MPL/2.0/.