-Because overlaps are checked and reported lazily, as described above, you need
-the <option>-fallow-overlapping-instances</option> in the module that <emphasis>calls</emphasis>
-the overloaded function, rather than in the module that <emphasis>defines</emphasis> it.</para>
-
+The willingness to be overlapped or incoherent is a property of
+the <emphasis>instance declaration</emphasis> itself, controlled by the
+presence or otherwise of the <option>-fallow-overlapping-instances</option>
+and <option>-fallow-incoherent-instances</option> flags when that mdodule is
+being defined. Neither flag is required in a module that imports and uses the
+instance declaration. Specifically, during the lookup process:
+<itemizedlist>
+<listitem><para>
+An instance declaration is ignored during the lookup process if (a) a more specific
+match is found, and (b) the instance declaration was compiled with
+<option>-fallow-overlapping-instances</option>. The flag setting for the
+more-specific instance does not matter.
+</para></listitem>
+<listitem><para>
+Suppose an instance declaration does not matche the constraint being looked up, but
+does unify with it, so that it might match when the constraint is further
+instantiated. Usually GHC will regard this as a reason for not committing to
+some other constraint. But if the instance declaration was compiled with
+<option>-fallow-incoherent-instances</option>, GHC will skip the "does-it-unify?"
+check for that declaration.
+</para></listitem>
+</itemizedlist>
+All this makes it possible for a library author to design a library that relies on
+overlapping instances without the library client having to know.
+</para>
+<para>The <option>-fallow-incoherent-instances</option> flag implies the
+<option>-fallow-overlapping-instances</option> flag, but not vice versa.