:- op(700, xfx, to).
direction("lr").

mma to mmb-xxx.
mmb to xxx-mmz.
mmz to mmb-xxx.
nna to nnb-xxx.
nnb to nnc-nnc.
nnc to nnz-nnz.
nnz to nnb-nnb.
xxx to xxx-xxx.