4 bells can be rung in several different orders - each sequence called a 'peal' .
Calling the bells A, B, C, and D, we have peals like : ABCD, ACDB, CADB etc.
'Ringing the changes' involves changing the order of the bells in a peal by a single 'swap' or 'exchange' of adjacent bells -
for example A B C D could become B A C D , using a Left exchange,
or A C B D , using a Middle exchange,
or, finally, A B D C , using a Right exchange .
The problem is to work your way through all the different peals, once and once only, using only L, M or R exchanges, and finishing up back at A B C D .
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