code: func (m *ReferenceMap) referenceMapKey(referrer dag.Vertex, addr addrs.Referenceable) string {
  	path := vertexReferencePath(referrer)
  	return m.mapKey(path, addr)
  }
docs: // referenceMapKey produces keys for the "edges" map. "referrer" is the vertex
  // that the reference is from, and "addr" is the address of the object being
  // referenced.
  //
  // The result is an opaque string that includes both the address of the given
  // object and the address of the module instance that object belongs to.
  //
  // Only GraphNodeSubPath implementations can be referrers, so this method will
  // panic if the given vertex does not implement that interface.