reserve X for BCI-algebra;
reserve n for Nat;
reserve x,y for Element of X;
reserve a,b for Element of AtomSet(X);
reserve m,n for Nat;
reserve i,j for Integer;

theorem
  x |^ 2 = x\x`
proof
  x |^ 2 = x|^(1+1) .= x\(x|^1)` by Def1;
  hence thesis by Th4;
end;
