Simplify can end up renaming external indices

[Krzmbrzl]

The following code snippet

auto input = parse_expr(L"-1 S{a_1,a_2;i_1,i_2}:N * f{i_3;i_2}:N * t{a_1,a_2;i_1,i_3}:N");
std::wcout << deparse_expr(input) << "\n";
auto output = simplify(input);

std::wcout << deparse_expr(input) << "\n" << deparse_expr(output) << "\n";

produces

-1 S{a_1,a_2;i_1,i_2}:N * f{i_3;i_2}:N * t{a_1,a_2;i_1,i_3}:N
-1 S{a_1,a_2;i_1,i_3}:N * f{i_2;i_3}:N * t{a_1,a_2;i_1,i_2}:N
-1 S{a_1,a_2;i_1,i_3}:N * f{i_2;i_3}:N * t{a_1,a_2;i_1,i_2}:N

Note that simplification has renamed the external index i_2 to i_3 and now uses i_2 as a contraction index.

Also, I am not sure if calling simplify on an expression is supposed to modify the handed-in expression in-place instead of only returning a simplified expression.