Include entity-less "scans.json" into an example of inheritance princ…

…iple

Clarifies #789

src/common-principles.md

@@ -739,9 +739,11 @@ A guide for using macros can be found at
             "sub-01_task-rest_acq-default_bold.nii.gz": "",
             "sub-01_task-rest_acq-longtr_bold.nii.gz": "",
             "sub-01_task-rest_acq-longtr_bold.json": "",
-            }
+            },
+        "sub-01_scans.tsv": "",
         },
     "task-rest_bold.json": "",
+    "scans.json": ""
     }
 ) }}
 
@@ -773,6 +775,8 @@ the value for field "`RepetitionTime`" is therefore overridden to the value `3.0
 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; corollary 3).
 
+A single `scans.json`, without any entity in the filename at the top level, is applicable to describe columns of the `sub-01_scans.tsv` and any other `_scans.tsv` potentially present in the dataset for other subjects.
+
 Example 2: Impermissible use of multiple metadata files at one directory level (rule 4)
 
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