Update operators section for structure addition

.gitignore

@@ -6,6 +6,8 @@ README.pdf
 *.old
 *.out
 
+*.output.idf
+
 doc/doc.md
 doc/doc.pdf
 

doc/code_samples/structure_addition_reference.nbem

@@ -0,0 +1,12 @@
+original_struct = {
+    'key': 'value',
+    'second key': 10
+}
+
+updated_struct = original_struct + { 'other key': 'passed along', 'second key': 20 }
+
+# This isn't a valid Version object, but go along with it.
+Version,
+  <updated_struct.'key'>,
+  <updated_struct.'second key'>,
+  <updated_struct.'other key'>;

doc/reference.inc.md

@@ -125,25 +125,43 @@ others are there for consistency, but the usage should be rare.
 
 ## Operators
 
+### Plus '`+`'
+- Addition for numeric types
+- String concatenation for string types
+    - `'Chiller ' + '1'`{.nbem} equals `'Chiller 1'`{.nbem}
+    - If using `'+'` operator with a string and a numeric, the
+      numeric is coerced to a string, and then the two strings are
+      concatenated. So `'Chiller ' + 1`{.nbem} (notice no quotes
+      around the number 1) becomes the string `'Chiller 1'` as you
+      would expect.
+- List concatenation for list types
+    - `[1, 2, 3] + [4, 5]` equals `[1, 2, 3, 4, 5]`
+- Structures - concatenate/update
+    - When two structures are used with the `+` operator, the keys from
+      both structures are combined into a new structure. If both
+      structures have the same key, then the value from the *second*
+      structure is used. This is how structures can be "updated" or
+      modified. An example is shown below.
+
+```neobem
+INCLUDE code_samples/structure_addition_reference.nbem
+```
+
+Compiling the above code results in:
+
+```neobem
+INCLUDE code_samples/structure_addition_reference.output.idf
+```
+
+
+### Other Algebraic Operators
+
+The following operators are only valid for numeric types.
+
 - Exponentiation: '`^`'
-    - Valid for numeric expressions only
-- Plus '`+`'
-    - Addition for numeric types
-    - String concatenation for string types
-        - `'Chiller ' + '1'`{.nbem} equals `'Chiller 1'`{.nbem}
-        - If using `'+'` operator with a string and a numeric, the
-          numeric is coerced to a string, and then the two strings are
-          concatenated. So `'Chiller ' + 1`{.nbem} (notice no quotes
-          around the number 1) becomes the string `'Chiller 1'` as you
-          would expect.
-    - List concatenation for list types
-        - `[1, 2, 3] + [4, 5]` equals `[1, 2, 3, 4, 5]`
 - Minus '`-`'
-    - Subtraction for numeric types
 - Multiplication '`*`'
-    - Multiplication for numeric types
 - Division '`/`'
-    - Division for numeric types
 
 
 ## Inline Data