remaining bits

Author: molpopgen

book/src/table_collection_adding_rows.md

@@ -27,6 +27,8 @@ See the [API docs](https://docs.rs/tskit) for more details and examples.
 
 ### Adding nodes using default values
 
+This section is more advanced and may be skipped during a first read.
+
 For some tables it may be common to input the same values over and over for some fields when adding rows.
 Let's take a look at how to use default values when adding rows to a node table.
 
@@ -81,8 +83,37 @@ This case is straightforward:
 
 ##### Case 2: default metadata
 
+TL;DR:
+
+* If table row defaults *include* metadata, you can run into use-after-move issues.
+  Fortunately, the compiler will catch this as an error.
+* The solution is for your metadata type to implement `Clone`.
+
 Consider the following case:
 
 ```rust, noplaygound, ignore
 {{#include ../../tests/book_table_collection.rs:node_defaults_with_some_metadata_default}}
 ```
+
+Imagine that the first row we add uses different metadata but all the other default values:
+
+```rust, noplaygound, ignore
+{{#include ../../tests/book_table_collection.rs:node_defaults_with_some_metadata_default_add_first_row}}
+```
+
+Nothing interesting has happened.
+However, let's take a look at what we need to do if our next row uses a non-default `population` field and the default metadata:
+
+```rust, noplaygound, ignore
+{{#include ../../tests/book_table_collection.rs:node_defaults_with_some_metadata_default_add_second_row}}
+```
+
+Note the call to `..defaults.clone()`.
+(For that call to compile, `NodeMetadata` must implement `Clone`!.) 
+Without that, our `defaults` instance would have *moved*, leading to a move-after-use compiler error when we add a third row:
+
+```rust, noplaygound, ignore
+{{#include ../../tests/book_table_collection.rs:node_defaults_with_some_metadata_default_add_third_row}}
+```
+
+