Merge pull request #3 from Lestropie/inheritance_unsetting_corollary

Inheritance principle: No unsetting as corollary

src/02-common-principles.md

@@ -563,15 +563,12 @@ for more information.
         accessing metadata associated with a data file MUST consider only the
         applicable file that is lowest in the filesystem hierarchy.
 
-    1.  For [JSON files](#key-value-files-dictionaries):
-
-        1.  Files are loaded from the top of the directory hierarchy downwards,
-            such that values from the top level are inherited by all data files
-            at lower levels to which it is applicable unless overridden
-            by a value for the same key present in another metadata file at a lower level
-            (though it is RECOMMENDED to minimise the extent of such overrides).
-
-        1.  There is no notion of "unsetting" a key/value pair.
+    1.  For [JSON files](#key-value-files-dictionaries), key-values are loaded
+        from files from the top of the directory hierarchy downwards, such that
+        key-values from the top level are inherited by all data files at lower
+        levels to which it is applicable unless overridden by a value for the
+        same key present in another metadata file at a lower level
+        (though it is RECOMMENDED to minimise the extent of such overrides).
 
 Corollaries:
 
@@ -585,6 +582,11 @@ Corollaries:
     across multiple data files, it is RECOMMENDED to store metadata in this
     way, rather than duplicating that metadata content across multiple metadata files.
 
+1.  Where multiple applicable JSON files are loaded as per rule 5.2, key-values can
+    only be overwritten by files lower in the filesystem hierarchy; the absence of
+    a key-value in a later file does not imply the "unsetting" of that field
+    (indeed removal of existing fields is not possible).
+
 Example 1: Demonstration of inheritance principle
 
 {{ MACROS___make_filetree_example(
@@ -623,10 +625,10 @@ metadata file `task-rest_bold.json` is read; file
 entity "`acq-longtr`" that is absent from the image path (rule 2.c). When reading image
 `sub-01/func/sub-01_task-rest_acq-longtr_bold.nii.gz`, metadata file
 `task-rest_bold.json` at the top level is read first, followed by file
-`sub-01/func/sub-01_task-rest_acq-longtr_bold.json` at the bottom level (rule 5.b.i);
+`sub-01/func/sub-01_task-rest_acq-longtr_bold.json` at the bottom level (rule 5.b);
 the value for field "`RepetitionTime`" is therefore overridden to the value `3000.0`.
 The value for field "`EchoTime`" remains applicable to that image, and is not unset by its
-absence in the metadata file at the lower level (rule 5.b.ii).
+absence in the metadata file at the lower level (rule 5.b; corollary 3).
 
 Example 2: Impermissible use of multiple metadata files at one directory level (rule 4)