A User's Guide to gringo, clasp, clingo, and iclingo

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5.4.3 Lookback Options The following options have an effect only if lookback techniques are turned on, that is, option --no-lookback is not used. --no-lookback Disable all lookback techniques. This option is included mainly for comparison purposes, and its use is not generally recommended. --restarts,-r n1[,n2,n3]|no Choose and parameterize a restart policy. If a single argument n1 is provided, clasp restarts search from scratch after a number of conflicts determined by a universal sequence [38], where n1 constitutes the base unit. If two arguments n1,n2 are specified, clasp runs a geometric sequence [11], restarting every n1 ∗ n2 i conflicts, where i is the number of restarts performed so far. Given three arguments n1,n2,n3, clasp repeats geometric restart sequence n1∗n2 i when it reaches an outer limit n3 ∗ n2 j [5], where j counts how often the outer limit has been hit so far. Finally, restarts are disabled via argument no. --local-restarts Count conflicts locally [49] (rather than globally) for deciding when to restart. --bounded-restarts Perform bounded restarts in answer set enumeration [21]. --reset-restarts Reset restart strategy whenever an answer set is found. --save-progress[=n] Use cached (rather than heuristic) decisions [45] if available. Cache decisions on backjumps > n. --shuffle,-s n1,n2 Shuffle internal data structures after n1 restarts (n1 = 0 standing for no shuffling) and then reshuffle every n2 restarts (n2 = 0 standing for no reshuffling). --deletion,-d n1[,n2,n3] Limit the number of learnt constraints to min{(c/n1) ∗ n2 i , c ∗ n3}, where c is the initial number of constraints and i is the number of restarts performed so far. --reduce-on-restart Delete a portion of learnt constraints after every restart. --estimate Base the initial limit of learnt constraints on an estimate of the problem’s complexity. --strengthen=bin|tern|all|no Check binary (with argument bin), binary and ternary (with argument tern), or all (with argument all) antecedents for self-subsumption [10] in order to strengthen a constraint to learn. Strengthening is disabled via argument no. --recursive-str Recursively apply strengthening, as proposed in [52]. 46
--loops=common|distinct|shared|no Learn loop nogood [22] per atom in an unfounded set [55] (with argument common), shrink unfounded set before learning another loop nogood (with argument distinct), learn loop formula [37] for atoms in an unfounded set (with argument shared), or do not record unfounded sets at all (with argument no). --contraction=n Temporarily truncate learnt constraints over more than n variables [48]. Let us note that switching the above search and lookback options can have dramatic effects (both positively and negatively) on the search performance of clasp. If performance bottlenecks are observed, it is worthwhile to give Vmtf and Vsids for --heuristic a try, in particular, when the program under consideration is huge but scarcely yields conflicts during search. Furthermore, we suggest trying the universal restart sequence [38] with different base units n1 or even disabling restarts (both via (long) option --restarts or its short form -r) in case that performance needs to be improved. For a brief overview on fine-tuning see [24]. Finally, let us consider the default command line of clasp: clasp 1 --verbose=1 --seed=1 --trans-ext=no --eq=5 --sat-prepro=no --lookahead=no --heuristic=Berkmin --deletion=3.0,1.1,3.0 --rand-freq=0.0 --rand-watches=yes --rand-prob=no --restarts=100,1.5 --shuffle=0,0 --strengthen=all --loops=common --contraction=250 Considering only the most significant defaults, numeric argument 1 instructs clasp to terminate immediately after finding an answer set, while --restarts=100,1.5 lets clasp apply a geometric restart policy. 6 Errors and Warnings This section explains the most frequent errors and warnings related to inappropriate inputs or command line options that, if they occur, lead to messages sent to the standard error stream. The difference between errors and warnings is that the former involve immediate termination, while the latter are hints pointing at possibly corrupt input that can still be processed further. In the below description of errors (Section 6.1) and warnings (Section 6.2), we refrain from attributing them to a particular one among the tools gringo, clasp, clingo, and iclingo, in view of the fact that they share a number of functionalities. 6.1 Errors We start our description with errors that may be encountered during grounding, where the following one indicates a syntax error in the input: ERROR: parsing failed: File:Line:Column: unexpected token: Token To correct this error, please investigate the line Line and check whether something looks strange there (like a missing period, an unmatched parenthesis, etc.). The next error occurs if an input program is not safe: 47
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Page 37 and 38: ation of plenty suboptimal answer s
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Page 41 and 42: --reify Output ground program in fo
Page 43 and 44: 5.4 clasp Options Stand-alone clasp
Page 45: --eq=n Run equivalence reasoning [2
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Page 55: Index Aggregates, 18 Average, #avg,

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