  We improve further the 2015 version of abcdSAT by various heuristics such as
at-least-one recently used strategy, learnt clause database approximation
reduction etc. Based on the requirement of different tracks at the SAT
Competition 2016, we develop three versions of abcdSAT: drup, inc and lim,
which participate in the competition of main (agile), incremental library and
no-limit track, respectively.
