title: How to determine whether a package is installed in elisp?
body: <p>I want to customize environment while the specific package is installed properly. How to check whether some package is installed in elisp?
  Something like this?:</p>
  
  <pre><code>(if (require 'ecb)
      (progn (setq ....))
    (message "ECB not installed!"))
  </code></pre>