title: Dr Racket problems with SICP
body: <p>I'm working through SICP. Currently, in the first chapter, I'm having problems getting Racket to let me redefine "primitives". For instance, I was under the impression that I should be able to arbitrarily do <code>(define + 5)</code> and that would be fine, or redefine the <code>sqrt</code> procedure. Instead, I get this:</p>
  
  <pre><code>define-values: cannot change constant variable: +
  </code></pre>
  
  <p>I have the language currently set to R5RS, which I was under the impression would take care of the compatibility issues with SICP.</p>