Is the subtype_diff function in CREATE TYPE only can be C function?

[ddme]

Hi all,

I notice that the CREATE TYPE syntax can specify subtype_diff function
CREATE TYPE name AS RANGE (
    SUBTYPE = subtype
    [ , SUBTYPE_OPCLASS = subtype_operator_class ]
    [ , COLLATION = collation ]
    [ , CANONICAL = canonical_function ]
    [ , SUBTYPE_DIFF = subtype_diff_function ] <————— here
    [ , MULTIRANGE_TYPE_NAME = multirange_type_name ]
)
And a example is
```sql
CREATE TYPE float8_range AS RANGE (subtype = float8, subtype_diff = float8mi);
```

I notice that float8mi is a C function, and I find the call_subtype_diff() in 
source code that it seems only can call C function.

I want to know

1. Can the subtype_diff function in CREATE TYPE be sql or plpgsql function?
2. How to call subtype_diff function? I know it related with GiST index, I need 
a example on how to trigger subtype_diff function.

What’s more,  I want to learn how Postgres kernel call subtype_diff function 
(in which source file or function), that will help me a lot.
 

Thank you all!