We will construct an algebraic weak factorisation system on the category of
01 substitution sets such that the R-algebras are precisely the Kan fibrations
together with a choice of Kan filling operation. The proof is based on Garner's
small object argument for algebraic weak factorization systems. In order to
ensure the proof is valid constructively, rather than applying the general
small object argument, we give a direct proof based on the same ideas. We use
this us to give an explanation why the J computation rule is absent from the
original cubical set model and suggest a way to fix this