How to call rule function in a different rule?

Issue

I'm trying to create a function that I can call from any of my rules.
I've created a DRL file that contains

package com.package;

function String hello(String name) {
    return "Hello "+name+"!";
}

In my rule I've got
import function com.package.hello;

LOG.info( hello(" Test "));

LOG works ok as I've imported it, but when I validate in business central I get
'Only a type can be imported' as one error and 'Error importing com.package.hello'

Note that function DRL file validates fine.

Is it possible to achieve this requirement? If so, how?