one argument position that is also polymorphic,
     and the actual data type(s) supplied for the polymorphic arguments
     determine the actual
     result type for that call.  For example, if there were not already
     an array subscripting mechanism, one could define a function that
     implements subscripting as <literal>subscript(anyarray, integer)
     returns anyelement</literal>.  This declaration constrains the actual first
     argument to be an array type, and allows the parser to infer the correct
     result type from the actual first argument's type.  Another example
     is that a function declared as <literal>f(anyarray) returns anyenum</literal>
     will only accept arrays of enum types.
    </para>

    <para>
     In most cases, the parser can infer the actual data type for a
     polymorphic result type from arguments that are of a different
     polymorphic type in the same family; for example <type>anyarray</type>
     can be deduced from <type>anyelement</type> or vice versa.
     An exception is that a
     polymorphic result of type <type>anyrange</type> requires an argument
     of type <type>anyrange</type>; it cannot be deduced
     from <type>anyarray</type> or <type>anyelement</type> arguments.  This
     is because there could be multiple range types with the same subtype.
    </para>

    <para>
     Note that <type>anynonarray</type> and <type>anyenum</type> do not represent
     separate type variables; they are the same type as
     <type>anyelement</type>, just with an additional constraint.  For
     example, declaring a function as <literal>f(anyelement, anyenum)</literal>
     is equivalent to declaring it as <literal>f(anyenum, anyenum)</literal>:
     both actual arguments have to be the same enum type.
    </para>

    <para>
     For the <quote>common</quote> family of polymorphic types, the
     matching and deduction rules work approximately the same as for
     the <quote>simple</quote> family, with one major difference: the
     actual types of the arguments need not be identical, so long as they
     can be implicitly cast to a single common type.  The common type is
     selected following the same rules as for <literal>UNION</literal> and
     related constructs (see <xref linkend="typeconv-union-case"/>).
     Selection of the common type considers the actual types
     of <type>anycompatible</type> and <type>anycompatiblenonarray</type>
     inputs, the array element types of <type>anycompatiblearray</type>
     inputs, the range subtypes of <type>anycompatiblerange</type> inputs,
     and the multirange subtypes of <type>anycompatiblemultirange</type>
     inputs.  If <type>anycompatiblenonarray</type> is present then the
     common type is required to be a non-array type.  Once a common type is
     identified, arguments in <type>anycompatible</type>
     and <type>anycompatiblenonarray</type> positions are automatically
     cast to that type, and arguments in <type>anycompatiblearray</type>
     positions are automatically cast to the array type for that type.
    </para>

    <para>
     Since there is no way to select a range type knowing only its subtype,
     use of <type>anycompatiblerange</type> and/or
     <type>anycompatiblemultirange</type> requires that all arguments declared
     with that type have the same actual range and/or multirange type, and that
     that type's subtype agree with the selected common type, so that no casting
     of the range values is required.  As with <type>anyrange</type> and
     <type>anymultirange</type>, use of <type>anycompatiblerange</type> and
     <type>anymultirange</type> as a function result type requires that there be
     an <type>anycompatiblerange</type> or <type>anycompatiblemultirange</type>
     argument.
    </para>

    <para>
     Notice that there is no <type>anycompatibleenum</type> type.  Such a
     type would not be very useful, since there normally are not any
     implicit casts to enum types, meaning