Fix some documentation

README.md

@@ -28,7 +28,7 @@ The `traccuracy` library has three main components: loaders, matchers, and metri
 Loaders load tracking graphs from other formats, such as the CTC format, into a [TrackingGraph](https://traccuracy.readthedocs.io/en/latest/autoapi/traccuracy/index.html#traccuracy.TrackingGraph) object.
 A TrackingGraph is a spatiotemporal graph.
 Nodes represent a single cell in a given time point, and are annotated with a time and a location.
-Edges point from a node representing a cell in time point `t` to the same cell or its daughter in `t+1`.
+Edges point forward in time from a node representing a cell in time point `t` to the same cell or its daughter in frames `t+1` (or beyond, to represent gap closing).
 To load TrackingGraphs from a custom format, you will likely need to implement a loader: see
 documentation [here](https://traccuracy.readthedocs.io/en/latest/autoapi/traccuracy/loaders/index.html#module-traccuracy.loaders) for more information.
 
@@ -55,4 +55,7 @@ pipelines, [documented here](https://traccuracy.readthedocs.io/en/latest/cli.htm
 : A single non-dividing cell tracked over time. In graph terms, this is the connected component of a track between divisions (daughter to next parent). Tracklets can also start or end with a non-dividing cell at the beginning and end of the captured time or if the track leaves the field of view.
 
 **Track**
-: A single cell and all of its progeny. In graph terms, a connected component including divisions.
+: A single cell and all of its progeny. In graph terms, a connected component including divisions.
+
+**Gap-Closing**
+: Also known as *frame-skipping*, these are edges that connect non-consecutive frames to signify a cell being occluded or missing for some frames, before the track continues.

src/traccuracy/_tracking_graph.py

@@ -81,6 +81,9 @@ class TrackingGraph:
     location (defaults to 'x' and 'y'). As in networkx, every cell must have a unique id, but these
     can be of any (hashable) type.
 
+    Edges typically connect nodes across consecutive frames, but gap closing or frame
+    skipping edges are valid, which connect nodes in frame t to nodes in frames beyond t+1.
+
     We provide common functions for accessing parts of the track graph, for example
     all nodes in a certain frame, or all previous or next edges for a given node.
     Additional functionality can be accessed by querying the stored networkx graph