title: What does "The following object is masked from 'package:xxx'" mean?
body: <p>When I load a package, I get a message stating that:</p>
  
  <pre><code>"The following object is masked from 'package:xxx'
  </code></pre>
  
  <p>For example, if I load <a href="http://www.rdocumentation.org/packages/testthat" rel="noreferrer"><code>testthat</code></a> then <a href="http://www.rdocumentation.org/packages/assertive" rel="noreferrer"><code>assertive</code></a>, I get the following:</p>
  
  <pre><code>library(testthat)
  library(assertive)  
  ## Attaching package: ‘assertive’
  ## 
  ## The following objects are masked from ‘package:testthat’:
  ## 
  ##     has_names, is_false, is_less_than, is_null, is_true
  </code></pre>
  
  <p>What does this message mean, and how do I prevent it?</p>