Function caller_binder_for 

pub fn caller_binder_for(
    handler: &Handler,
    actors: &HashMap<String, ActorDecl>,
) -> Option<String>

v0.54: the binder of a cross-context on call … by c: Caller handler that captures a live CallerId (the calling context’s name, Q7). None unless the handler binds an identity whose contract is CallerId — i.e. the Caller prelude actor (the only source of CallerId). A binder-less on call (or one inheriting the Caller default) captures nothing and is unaffected.