title: rnbinom in R returns error that says optional argument is missing
body: <p>I am trying to reset the default arguments in the <code>rnbinom</code> function with the following example code:</p>
  
  <pre><code>params &lt;- c("size" = 1, "mu" = 1)
  formals(rnbinom)[names(params)] &lt;- params
  rnbinom(n = 10)
  </code></pre>
  
  <p>It returns the following:</p>
  
  <p><code>Error in rnbinom(n = 10) : argument "prob" is missing, with no default</code></p>
  
  <p>If I set the defaults with this code:</p>
  
  <pre><code>params &lt;- c("size" = 1, "prob" = .5)
  formals(rnbinom)[names(params)] &lt;- params
  rnbinom(n = 10)
  </code></pre>
  
  <p>The function works correctly. The documentation specifies that you can set mu or prob with size. How is rbinom evaluating this function so that the order of arguments set with formals has an effect?</p>