Finding the photos that have NOT been keyworded?

[Mamusworld]

In LIghtroom, can anyone tell me how to sort for the photos that have NOT been keyworded? I'm sure I found it once and now I can't.  I've looked every way I can think of. 

[John Waller · Accepted solution]

One method: Advanced Smart Collections In Lightroom 5 « Julieanne Kost's Blog

[Per Berntsen]

Another method:

On the filter bar, choose Metadata, then click on one of the headings, and choose Keyword from the menu, select None.