code: func Caller() *Call {
  	scope, entry := findScope()
  	if scope == nil {
  		return nil
  	}
  	return &Call{
  		scope: scope,
  		entry: entry,
  	}
  }
docs: // Caller will search the stack, looking for a Terst testing scope. If a scope
  // is found, then Caller returns a Call for logging errors, accessing testing.T, etc.
  // If no scope is found, Caller returns nil.