[plt-scheme] Re: hash-has-key?

On 28 Mar, 01:14, Henk Boom <h... at henk.ca> wrote:
> Hi, I was looking for a way to test whether or not a hash table has a
> mapping for a given key. I was surprised not to find anything in the
> docs (...)

Interesting. I found hash-has-key? in the PLT Reference
(http://docs.plt-scheme.org/reference/hashtables.html), but I cannot
use it:

Welcome to DrScheme, version 4.1.4 [3m].
Language: Module; memory limit: 128 megabytes.
> (define a (make-hash))
> (hash-has-key? a 'key)
. . reference to undefined identifier: hash-has-key?

What's wrong? (I'm new to Scheme and to PLT Scheme.)