docs(3p/nix/libexpr): Add comment on ExprSelect

third_party/nix/src/libexpr/nixexpr.hh

@@ -147,6 +147,13 @@ struct ExprVar : Expr {
   Value* maybeThunk(EvalState& state, Env& env);
 };
 
+// [tazjin] I *think* that this struct describes the syntactic
+// construct for "selecting" something out of an attribute set, e.g.
+// `a.b.c` => ExprSelect{"b", "c"}.
+//
+// Each path element has got a pointer to an expression, which seems
+// to be the thing preceding its period, but afaict that is only set
+// for the first one in a path.
 struct ExprSelect : Expr {
   Pos pos;
   Expr *e, *def;