title: All valid values of an argument of a function in R
body: <p>Suppose we have an R function whose arguments must be selected out of a finite set of elements. Like <code>qplot(..., geom="")</code>. And <code>geom</code> can take only some values, like <code>bar</code> or <code>point</code>.</p>
  
  <p>How can I find out all the valid values the argument of a given function may take? Apart from docs or Internet, which often miss all possible values. Perhaps, some R function can help?</p>