Document expectations for column val cardinality

input/pagecontent/StructureDefinition-ViewDefinition-notes.md

@@ -497,6 +497,9 @@ that any `unionAll` branches produce the same columns.
 * `S`: a Selection Structure
 * `N`: a Node (element) from a FHIR resource
 
+**Errors**
+* Multiple values found but not expected for column
+
 **Emits:** One output row at a time
 
 **Purpose:** This step will take sets of "partial rows" (i.e., sets of incomplete column bindings from the various clauses of `V`) and combine them to emit complete rows. For example, if there are two sets of partial rows:
@@ -523,8 +526,12 @@ Then the Cartesian product of these sets consists of four complete rows:
 2. For each element `f` of `foci`
     1. Initialize an empty list `parts` (each element of `parts` will be a list of partial rows)
     2. Process Columns:
-        * For each column `col` of `S.column`
-            1. Define `b` as a row whose column named `col.name` takes the value `fhirpath(col.path, f)`
+        * For each column `col` of `S.column`, define `val` as  `fhirpath(col.path, f)`
+            1. Define `b` as a row whose column named `col.name` takes the value
+                * If `val` was the empty set: `null`
+                * Else if `val` has a single element `e`: `e`
+                * Else if `col.collection` is true: `val`
+                * Else: throw "Multiple values found but not expected for column"
             2. Append `[b]` to `parts`
             * (Note: append a list so the final element of `parts` is now a list containing the single row `b`).
     3. Process Selects: