+ // After having subs'ed all children, this method subs'es one final
+ // level, but only if the intermediate result is a container! This is
+ // because if the intermediate result has eval'ed to a non-container a
+ // last level substitution would be wrong, as this example involving a
+ // function f and its inverse f^-1 shows:
+ // f(x).subs(x==f^-1(x))
+ // -> f(f^-1(x)) [subschildren]
+ // -> x [eval] /* must not subs(x==f^-1(x))! */