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(*  Levy's cube lemma :                                                   *)
(*  (U,V,UV,VU:redexes)  (residuals U V UV) -> (residuals V U VU) ->      *)
(*  (W,WU,WV,WUV:redexes)(residuals W U WU) -> (residuals WU VU WUV) ->   *)
(*                       (residuals W V WV) -> (residuals WV UV WUV).     *)
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(* 3-dimensional paving diagram *)
Lemma paving :
forall U V W WU WV : redexes,
residuals W U WU ->
residuals W V WV ->
exists UV : redexes,
(exists VU : redexes,
(exists WUV : redexes, residuals WU VU WUV /\ residuals WV UV WUV)).