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 Th5:
  x |^ -1 = x`
proof
  x |^ (-1)= BCI-power(X).(x`,|.-1 .|) by Def2;
  then x |^ (-1)= BCI-power(X).(x`,-(-1)) by ABSVALUE:def 1;
  then x |^ (-1)= x`|^ 1;
  hence thesis by Th4;
end;
