title: What does a double colon followed by an equals sign (::=) mean in programming documentation?
body: <p>What does <code>::=</code> mean in programming documentation?
  <br />For example in the <a href="http://www.lua.org/manual/5.2/manual.html#3.2" rel="noreferrer">Lua documentation</a>: or in <a href="http://docs.python.org/release/2.7.1/reference/lexical_analysis.html#identifiers" rel="noreferrer">Python documentation</a>.</p>