How Do You Troubleshoot the Ansible Message “ERROR! the field hosts is required but was not set”?

Problem scenario
You run an ansible-playbook command, but you get "ERROR! the field hosts is required but was not set". What should you do?

Solution
Find the hyphens beneath the top "- name" stanza. Eliminate them. For example, if your playbook looks like this:

- name: Foobar
  hosts: localhost
- vars:
       goodone: "blahblah"
-tasks:
  …

Make it look like this:

- name: Foobar
  hosts: localhost
  vars:
       goodone: "blahblah"
  tasks:
 …

Now re-run it. (The root cause of this problem in all likelihood is syntax.)