How to reuse scripts withing script collections

[BHerbold]

Hello,
As example I have following Groovy script within an iFlow:

def Message setCustomHeaderProcessToDelete(Message message) {
    return setCustomHeaderProcess(message, "Delete")
}
def Message setCustomHeaderProcessToAdd(Message message) {
    return setCustomHeaderProcess(message, "Add")
}
def Message setCustomHeaderProcessToModify(Message message) {
    return setCustomHeaderProcess(message, "Modify")
}
def Message setCustomHeaderProcess(Message message, process) {
 	def messageLog = messageLogFactory.getMessageLog(message);
    if(messageLog == null){ 
        return message; 
    } 
    messageLog.addCustomHeaderProperty("PROCESS", (String) process);
    return message 
}

I would like to move this functions to a script collection. As you can see several function call setCustomHeaderProcess.
How do I have to store setCustomHeaderProcess?
How do I have to call it from a function, by example setCustomHeaderProcessToModify?

Thanks
Bernd