blank predicates

I am working on a information mapping system (aka mind maps)

And I want to have two nodes related to each other

#Alice R #Bob

In the general sense, the type of relationship (predicate) R is not really
known at the time of creation.  My software currently does not allow the
labeling of edges is the reason (but hopefully in future it will)

I need a way to relate Alice to Bob but I dont have a URI for a predicate.

Is there something that can operate as a "blank predicate"?

Or some existing relations that simply says that two entities or linked /
related, without yet knowing how they are related?