Parses abstract argumentation frameworks in the CNF/Dimacs Format used for the
SAT solver competition (see also http://people.sc.fsu.edu/~jburkardt/data/cnf/cnf.html).
It is given by the following BNF (start symbol is S):
S ::== COMMENTS "\n" PROBLEM "\n" ATTACKS
COMMENTS ::== ("c" COMMENT "\n")*
PROBLEM ::== "p af " INT INT
ATTACKS ::== (INT " -" INT " 0\n")*
where "COMMENT" represents any string, and INT is a positive natural number. Lines beginning
with "c" are comment lines and are ignored by the parser. The line beginning with "p af" is the
problem definition, it is followed by two positive integers where the first is the number
of arguments and the second is the number of attacks in the given framework. After the problem
definition there is one line for each attack. The first integer in these lines describe the attacker
of the attack and the second the attacked (the attacked is also prefixed by a minus sign "-"). Each
line of an attack ends with "0" and a line break.