title: What does "plus colon" ("+:") mean in shell script expressions?
body: <p>What does this mean?</p>
  
  <pre><code>if ${ac_cv_lib_lept_pixCreate+:} false; then :
    $as_echo_n "(cached) " &gt;&amp;6
  else
    ac_check_lib_save_LIBS=$LIBS
  </code></pre>
  
  <p>Looks like <code>ac_cv_lib_lept_pixCreate</code> is some variable, so what does <code>+:</code> mean?</p>
  
  <p>Where to learn complete syntax of curly bracket expressions? What is the name of this syntax?</p>