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Given a tile of a tiling, its Conway signature is an infinite-to-the-left sequence of symbols taken in {0,1,2,3,4,5,6,7}
that records the subdivision history that originated it.
Most recent subdivision history is recorded to the right, in particular the last symbol of the signature is 0 iff we are focusing on
a mystic tile.
 The meaning of the symbols is clarified in the image on the left showing the subdivision process for a "spectre" tile. In a signature we do not allow for the consecutive pair of symbols ...03... The notation "[w]p." denotes the periodic signature "...wwwwp." where w is any nonempty finite sequence of symbols and p is a possibly empty finite sequence of symbols.  | 
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        [40]. 
       Core of the spectre tile.  Tripod tiling
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        [40]. 
       Subdivision of the previous.  Tripod tiling.
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     [0]. and [3]. twice union of [0]. and two copies of [3].Name: Hydra 2  | 
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     [1]. twice union of two copies of [1].
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     [5]. and [3]. union of [5]. and [3].Name: Hydra 1  | 
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     [4]. three times union of three copies of [4].
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     [6]. three times union of three copies of [6].
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