if you get something from x[6j] it's mapping.
Yeah - good point.
It's possible to implement a mapping type that doesn't rely on hashing the keys - for example, a binary search tree.
I think your best bet is just relying on the fact that sequence types won't work if you pass them something that isn't an integer. (Although if someone's using a custom sequence type that does something different in that case (fairly pathological situation), you'll still have a problem.)
Commenting by HaloScan
