In Being Popular, PG says:<p><pre><code> One solution here might be to design systems so that interfaces are
horizontal instead of vertical-- so that modules are always vertically
stacked strata of abstraction. Then the interface will tend to be owned by
one of them. The lower of two levels will either be a language in which
the upper is written, in which case the lower level will own the interface,
or it will be a slave, in which case the interface can be dictated by the
upper level.
</code></pre>
I think I understand what a language interface looks like, but I do not understand what a slave interface looks like. Can anyone here provide any good examples?