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Fig. 6. Definition of track angle, track distance and crab side, the three
parameters used to describe the dummy trajectory relative to the burrow and
the crab's home vector. `Crab side' measures whether the dummy track
intercepts the line defined by the crab's home vector on the crab side of the
burrow (crab side = 1) or on the opposite side (crab side = 0). The variable
is needed because there are always two paths that have a particular
combination of track angle and track distance.