RC4: Mine - Barrel as trap

[HaJo]

In the mission Wellington/mine, a soldier can enter a square
with a barrel from above.
Once in the barrel, he is trapped and cannot move out,
see screenshots at issue 1535195 on sf.net,
https://sourceforge.net/tracker/index.php?func=detail&aid=1535195&group_id=157793&atid=805242