How to delete a non-empty directory using the Dir class?

Dir.delete("/usr/local/var/lib/trisul/CONTEXT0/meters/oper/SLICE.9stMxh")  

causes this error:

Directory not empty - /usr/local/var/lib/trisul/CONTEXT0/meters/oper/SLICE.9stMxh

How to delete a directory even when it still contains files?