The definitions part says:
Given an *autoloadable class name* of Acme\Log\Writer\FileWriter, a *namespace
prefix* may be Acme\, Acme\Log\, or Acme\Log\Writer\.


2013/10/23 AmyStephen <notifications@github.com>

> In proposed/psr-4-autoloader/psr-4-autoloader.md:
>
> >
> > -  2. For every combination of _namespace prefix_ and _directory_ found,
> > -  take the _relative class name_  replace every _namespace separator_ in
> > -  it with a directory separator. Append the ".php" suffix, and append the result
> > -  to the _base path_, to obtain a _file path_.
> > -
> > -  3. If any of the _file paths_ obtained this way points to an existing file,
> > -  then include or require exactly one of these files.
> > +  1. One of the _namespace prefixes_ of the class MUST correspond to one of the
>
> A class exists in a single namespace. There is sub-namespace<http://www.php.net/manual/en/language.namespaces.nested.php>to help describe a namespace with a built in hierarchy. I honestly don't
> understand that sentence, part of it is that it's in a passive tense, but
> mainly those are not PHP terms so I have no context.
>
> —
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> .
>