  The original Halpern-Pearl definition of causality [Halpern and Pearl, 2001]
was updated in the journal version of the paper [Halpern and Pearl, 2005] to
deal with some problems pointed out by Hopkins and Pearl [2003]. Here the
definition is modified yet again, in a way that (a) leads to a simpler
definition, (b) handles the problems pointed out by Hopkins and Pearl, and many
others, (c) gives reasonable answers (that agree with those of the original and
updated definition) in the standard problematic examples of causality, and (d)
has lower complexity than either the original or updated definitions.
