Connectables

It's not connectables that you want to explore, it's Ports. Connectables are a representational thing, ports are the conceptual things, and MERL deals (primarily) with conceptual things.

So you want to go through all Masters, and for each one go through its MasterSlave relationships. Actually, we'll walk first just into the Master role, and from there go on an excursion to the Slave role and Slave ("Slave 1"), and then again on an excursion to the MyPort port:

foreach .Master {
do ~Master {
    do ~Slave.Slave { id }
    ' on '
    do #MyPort { id }
    newline
}
}

It's generally best to navigate into roles rather than relationships. In this case it doesn't matter much, but if both roles were of the same type (e.g. Association End in UML), if you go to the relationship and look at the roles, they all look the same so it's hard to know which way you want to go next.

Similarly you never want to navigate on from a port. You can pop into one on an excursion, but asking for all the roles connected to "Port 1" is pretty meaningless: "Port 1" is defined in the metamodel, so every Master object has the identical same "Port 1", so you would then be following ALL roles connected to ANY Master object on Port 1.