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
  a`|^(-n) = (a|^(-n))`
proof
  reconsider c =a` as Element of AtomSet(X) by BCIALG_1:34;
  c|^(-n) = c`|^n by Th10
    .= (c|^n)` by Th17
    .= (a|^(-n))` by Th10;
  hence thesis;
end;
