Add change 27 to instructions

how_to/Change_27.md

@@ -0,0 +1,21 @@
+- k: Iterate commits and parents
+
+In addition to printing the refs, we'll also print all OIDs that are reachable
+from those refs. We'll create `iter_commits_and_parents`, which is a generator
+that returns all commits that it can reach from a given set of OIDs.
+
+Note that `iter_commits_and_parents` will return an OID once, even if it's
+reachable from multiple refs. Here, for example:
+```
+o<----o<----o<----o<----@<----@<----@
+^                  \                ^
+first commit        -<--$<----$     refs/tags/tag1
+                              ^
+                              refs/tags/tag2
+```
+
+We can reach the first commit by following the parents of *tag1* or by following
+the parents of *tag2*. Yet if we call `iter_commits_and_parents({tag1, tag2})`,
+the first commit will be yielded only once. This property will be useful later.
+
+(Note that nothing is visualized yet, we're preparing for that.)