title: racket scheme redefinition of constants
body: <p>I am currently using racket scheme with DrRacket as the editor for some exploratory programming.  When I try and redefine a function I get the following message:</p>
  
  <pre><code>define-values: cannot re-define a constant: png-&gt;byte-list
  </code></pre>
  
  <p>Now I used to accept that this was a property of the language and was frustrated by it but kind of accepted it.  Having read <a href="https://stackoverflow.com/questions/624794/how-can-you-re-define-a-constant-identifier-in-drscheme">How can you re-define a constant identifier in DrScheme?</a> I can see that I don't necessarily have to accept this limitation.</p>
  
  <p>My questions are as follows:</p>
  
  <ol>
  <li>Is this feature only available in the R5RS language?  Is there a way to switch this feature off for an arbitrary language?</li>
  <li>Alternatively if I switch the R5RS language will this have an impact on me using the modules I'm using at the moment: racket/gui/base and web-server/insta?</li>
  </ol>