1.1.2.  Abstraction-based heuristics

Historically, abstraction reﬁnement was the dominating abstraction method in planning, but it has been largely replaced 
by  abstraction heuristics.  The  heuristic  search  approach  has  proven  very  successful  both  in  planning  and  in  search,  and 
a  large  number  of  well-performing  domain-independent  heuristics  have  been  invented  (see  the  paper  by  Helmert  and 
Domshlak [47] for  a  comprehensive  survey  and  comparison).  In  abstraction  heuristics,  the  abstract  solution  is  not  used 
directly, but the length (or cost) of it is used as a heuristic value to estimate the length (or cost) of an optimal solution in 
the ground instance. The abstract solutions are thus only used indirectly to guide a heuristic search algorithm. This method 
dates back to Gaschnig [36] and to Guida and Somalvico [43].
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