Capture by surrounding a piece (custodian/enclosure)

In some games like hnefatafl and its variants, pieces don't capture by moving into an enemy piece like chess, instead a piece must be surrounded to capture it. See [hnefatafl rules](http://tafl.cyningstan.com/page/27/capturing-the-enemy).

There are lots of complicated variations:
* Sometimes surrounding on two sides is needed (custodianship) and sometimes four (enclosure). This can vary between piece types in the game.
* Some squares can sometimes act as part of the capturing group.
* Sometimes all piece types can participate in capture, and sometimes it's only the pawn equivalents.