[plt-scheme] hash-ref 'manipulating' mutable hash-tables?

Please read the full sentence.

Chongkai

Stephen De Gabrielle wrote:
> Hi,
>
> "Caveats concerning concurrent modification: A mutable hash table can
> be manipulated with hash-ref,"
> --  http://docs.plt-scheme.org/reference/hashtables.html
>
> Is this right? How does hash-ref change a hash table?  (do mutable
> ht's do some sort of 'query optimisation' that I'm not aware of? if
> so; does this make them faster but less safe than immutable ht's?)
>
>
> Cheers,
>
> Stephen
>
>