What does "The following object is masked from 'package:xxx'" mean?
<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>