autfilt: add --remove-unused-ap

Part of #170.

* bin/autfilt.cc: Here.
* tests/core/remprop.test: Test it.
* NEWS: Mention it.

tests/core/remprop.test

@@ -1,6 +1,6 @@
 #! /bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2015 Laboratoire de Recherche et Développement de
+# Copyright (C) 2015, 2016 Laboratoire de Recherche et Développement de
 # l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -95,3 +95,40 @@ diff out expected
 
 autfilt -H --remove-ap=a==1 automaton 2>stderr && exit 1
 grep "autfilt: unexpected '=' at position 2 in 'a==1'" stderr
+
+
+cat >automaton <<EOF
+HOA: v1
+States: 1
+Start: 0
+AP: 3 "a" "b" "c"
+acc-name: Buchi
+Acceptance: 1 Inf(0)
+properties: trans-labels explicit-labels state-acc colored
+properties: deterministic
+--BODY--
+State: 0 {0}
+[1] 0
+--END--
+EOF
+
+autfilt automaton > output
+diff automaton output
+
+cat >expect <<EOF
+HOA: v1
+States: 1
+Start: 0
+AP: 1 "b"
+acc-name: Buchi
+Acceptance: 1 Inf(0)
+properties: trans-labels explicit-labels state-acc colored
+properties: deterministic
+--BODY--
+State: 0 {0}
+[0] 0
+--END--
+EOF
+
+autfilt --remove-unused-ap automaton > output
+diff output expect