fix:set difference instead of symmetric difference

alib2/src/automaton/FSM/CompactNFA.cpp

alib2/src/automaton/FSM/ExtendedNFA.cpp

alib2/src/automaton/PDA/PDA.cpp

alib2/src/automaton/common/BlankSymbolInputTapeAlphabet.cpp

alib2/src/automaton/common/InputAlphabet.cpp

alib2/src/automaton/common/MultiInitialStates.cpp

alib2/src/automaton/common/States.cpp

alib2/src/regexp/RegExp.cpp