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grep!$s{$_}&!($s&8&&~$s&$_&7)&!(~$s&8&&$s&~$_&7)&&($z=$s^$_)&8&&(($z&=
+7)<3|$z==4),15,grep$_%3,1..14

0-8    11-1    7-15
 \    /    \    /
  10-2      13-5
      \    /
       14-4

grep
  !$s{$_}              # we have not been at $_ already
...
  &&($z=$s^$_)&8       # the boat moves
  &&(($z&=7)<3|$z==4), # with at most one passenger
  15,grep$_%3,1..14    # possible choices for moves