title: Assign only if not already defined in Julia
body: <p>Is there simple Julia syntax for assigning to a variable only if it is undefined (or falsy)?  I mean something like Ruby's <code>x ||= NEW_VALUE</code>.  I have tried <code>x || x=NEW_VALUE</code> but it throws an error.  Barring easy syntax, what function can I use to check if a variable is defined?</p>