title: How to include (source) R script in other scripts
body: <p>I've created a utility R script, util.R, which I want to use from other scripts in my project.
  What is the proper way to ensure that the function this script defines are available to function in my other scripts?</p>
  
  <p>I'm looking for something similar to the <code>require</code> function, that loads a package only if it has not been loaded yet. I don't want to call <code>source("util.R")</code> because that will load the script every time it is called.</p>
  
  <p>I know that I will get some answers telling me to create a package, as in <a href="https://stackoverflow.com/q/2284446/761313">Organizing R Source Code</a> :)
  But I'm not creating something that will be used elsewhere, it is just a standalone project.</p>