title: Check if the argument passed but does not exist
body: <p>In my function, I would like to determine the existence of variable passed as argument, for example:</p>
  
  <pre><code>test &lt;- function(input) {
    out="nothing";
    if (exists(input)) {out="input exists";} else {out="input does not exist";}
    return(out);
  }
  
  myvar=0;
  
  test(myvar);     # I expect "input exists"
  test(blablabla); # I expect "input does not exist"
  </code></pre>
  
  <p>Is there any way to implement such behavior?</p>