Example of CliBuilder with "optionalArg: true"?

[David M. Karr]

Because someone asked me a question about it, I was looking at 
CliBuilder, for a scenario where "optionalArg" is true, and to get the 
value of the option, or detect that it's not present.  I've looked at 
the groovydoc, and CliBuilderTest, but it's not clear to me how this works.