title: base::assign(".ptime", proc.time(), pos = "CheckExEnv") ERROR when using devtools::check
body: <p>I am doing the R CMD check for my package using <code>devtools::check</code> and I encountered the same <code>ERROR</code>(see bellow) discussed <a href="https://stackoverflow.com/questions/17684026/check-new-r-package">here</a>. 
  I tried to do what was suggested there: I added a tag of <code>#'@export</code> before the <code>#'@example</code> in my <code>prep.R</code> code, and I also added <code>export(prep)</code> in <code>NAMESPACE</code>. However I still get the same error.</p>
  
  <p>Does anyone knows how can I solve this?</p>
  
  <p>Any help will be greatly appreciated</p>
  
  <p>Ayala </p>
  
  <pre><code>* checking R/sysdata.rda ... OK
  * checking examples ... ERROR
  Running examples in 'prepdat-Ex.R' failed
  The error most likely occurred in:
  
  &gt; base::assign(".ptime", proc.time(), pos = "CheckExEnv")
  &gt; ### Name: prep
  &gt; ### Title: Aggregate Long Format Data According to Grouping Variables and
  &gt; ###   Generate a Number of Measures for Each Cell in the Aggregated Data
  &gt; ###   for Further Analysis
  &gt; ### Aliases: prep
  &gt; 
  &gt; ### ** Examples
  &gt; 
  &gt; data(stroopdata)
  &gt; x1 &lt;- prep(
  +          dataset = stroopdata
  +          , file_name = NULL
  +          , id = "subject"
  </code></pre>