resolved
     to specific data types when a query calling a polymorphic function is
     parsed.  When there is more than one polymorphic argument, the actual
     data types of the input values must match up as described below.  If the
     function's result type is polymorphic, or it has output parameters of
     polymorphic types, the types of those results are deduced from the
     actual types of the polymorphic inputs as described below.
    </para>

    <para>
     For the <quote>simple</quote> family of polymorphic types, the
     matching and deduction rules work like this:
    </para>

    <para>
     Each position (either argument or return value) declared as
     <type>anyelement</type> is allowed to have any specific actual
     data type, but in any given call they must all be the
     <emphasis>same</emphasis> actual type. Each
     position declared as <type>anyarray</type> can have any array data type,
     but similarly they must all be the same type.  And similarly,
     positions declared as <type>anyrange</type> must all be the same range
     type.  Likewise for <type>anymultirange</type>.
    </para>

    <para>
     Furthermore, if there are
     positions declared <type>anyarray</type> and others declared
     <type>anyelement</type>, the actual array type in the
     <type>anyarray</type> positions must be an array whose elements are
     the same type appearing in the <type>anyelement</type> positions.
     <type>anynonarray</type> is treated exactly the same as <type>anyelement</type>,
     but adds the additional constraint that the actual type must not be
     an array type.
     <type>anyenum</type> is treated exactly the same as <type>anyelement</type>,
     but adds the additional constraint that the actual type must
     be an enum type.
    </para>

    <para>
     Similarly, if there are positions declared <type>anyrange</type>
     and others declared <type>anyelement</type> or <type>anyarray</type>,
     the actual range type in the <type>anyrange</type> positions must be a
     range whose subtype is the same type appearing in
     the <type>anyelement</type> positions and the same as the element type
     of the <type>anyarray</type> positions.
     If there are positions declared <type>anymultirange</type>,
     their actual multirange type must contain ranges matching parameters declared
     <type>anyrange</type> and base elements matching parameters declared
     <type>anyelement</type> and <type>anyarray</type>.
    </para>

    <para>
     Thus, when more than one argument position is declared with a polymorphic
     type, the net effect is that only certain combinations of actual argument
     types are allowed.  For example, a function declared as
     <literal>equal(anyelement, anyelement)</literal> will take any two input values,
     so long as they are of the same data type.
    </para>

    <para>
     When the return value of a function is declared as a polymorphic type,
     there must be at least 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>