Merges

preserves.md

@@ -174,7 +174,7 @@ URL, compared according to
 usually be represented as ordinary `Value`s, in which case the
 ordinary rules for comparing `Value`s will apply.
 
-## Examples
+## <a id="examples"></a>Appendix. Examples
 
 The definitions above are independent of any particular concrete syntax.
 The examples of `Value`s that follow are written using [the Preserves
@@ -358,5 +358,40 @@ encodes to binary as follows:
       84
     84
 
+## Appendix. Merging Values
+
+The *merge* of two `Value`s is a combination of the two values that includes all information
+from each that is missing from the other. If the values are incompatible, they have no merge.
+
+ - the merge of two `Atom`s has no value if they are not [equal](#equivalence); otherwise, it
+   has value equal to (an arbitrary) one of the atoms.
+
+ - the merge of two `Embedded`s depends on the interpretation of the embedded values, and so is
+   implementation-defined.
+
+ - the merge of two `Compound`s is:
+
+   - if both are `Sequence`s, let `n` be the minimum of the lengths of the two sequences. If
+     every merge of corresponding positions up to `n` in the sequences is defined, the result
+     is defined, with elements merged up to position `n` and simply copied from the longer
+     sequence from position `n` onward; otherwise, it is undefined.
+
+   - if both are `Record`s, the `Record` with the merge of the two input records' labels as its
+     label and the merge of the inputs' field sequences as its fields;
+
+   - if both are `Dictionary`s, if every merge of values associated with keys common to both
+     inputs is defined, the result is defined, with merged values at common keys and simply
+     copied from either side for keys unique to that side; otherwise, it is undefined.
+
+ - Otherwise, the merge is undefined.
+
+**Examples.**
+
+ - `merge [1, [2], 3] [1, [2, 99], 3, 4, 5] = merge [1, [2, 99], 3, 4, 5]`
+ - `merge [1, 2, 3] [1, 5, 3] = ⊥`
+ - `merge #{a, b, c} #{a, b, c} = ⊥`
+ - `merge {a: 1, b: [2]} {b: [2, 99] c: 3} = {a: 1, b: [2, 99], c: 3}`
+ - `merge {a: 1, b: [2]} {a: 5, b: [2]} = ⊥`
+
 <!-- Heading to visually offset the footnotes from the main document: -->
 ## Notes