Overloaded SubPort operations will have unexpected behaviour

[zenna]

Currently if you do x + y where x and y are subports, a new AddArrow will be created and added to the parent composition of x and y, and links will be made from x and y to the in_ports of the subarrow.

However, there are several cases we haven't accounted for which will simply break or give undesired or unexpected behaviour:

• x and y have different parents, should this be an undefined operation or should we attempt a kind of merge
• x or y are dst SubPorts rather than src. Should this be undefined or should we go back to the source. Should we delete the current edge