reserve X for BCI-algebra;
reserve x,y,z,u,a,b for Element of X;
reserve IT for non empty Subset of X;

theorem
  X is weakly-positive-implicative BCI-algebra implies for x,y being
  Element of X holds (x\(x\y))\(y\x)=((y\(y\x))\(y\x))\(x\y) by Lm24;
