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 alternative implies y\(y\(x\(x\y))) = y
proof
  assume X is alternative;
  then y\(y\(x\(x\y)))=y\(y\y) by Th76
    .=y\0.X by Def5
    .= y by Th2;
  hence thesis;
end;
