n of the method, this puts you in one of five situations:
<ol>
<li>You actually never return nil.</li>
<li>You do return nil sometimes, and callers are supposed to handle that. This
includes cases where your method is documented to return nil given certain
inputs.</li>
<li>You return nil based on some external condition (such as an out-of-memory
error), but the client can't do anything about it either.</li>
<li>You return nil only when the caller passes input documented to be invalid.
That means it's the cl