 they aren't copied so you shouldn't modify them in-place.
  </p>

  <p>
    Sources can be both files and directories, however it can be useful to use
    globs for sources e.g. to include all files with a given extension. See the
    <a class="copy-link" href="/lexicon.html#please-builtins">built ins</a>
    for more information on this.
  </p>

  <p>
    Sources can be referred through the
    <code class="code">$SRCS</code> environment variable. If there's only one,
    <code class="code">$SRC</code> wi