add extra_slots metamodel slot

linkml_model/model/schema/meta.yaml

@@ -1351,7 +1351,73 @@ slots:
     range: boolean
     description: if true then induced/mangled slot names are not created for class_usage and attributes
     status: testing
-
+
+  extra_slots:
+    description: |
+      How a class instance handles extra data not specified in the class definition.
+      Note that this does *not* define the constraints that are placed on additional slots defined by inheriting classes.
+
+      Possible values:
+      - `allowed: true` - allow all additional data
+      - `allowed: false` (or `allowed:` or `allowed: null` while `range_expression` is `null`) - 
+        forbid all additional data (default) 
+      - `range_expression: ...`  - allow additional data if it matches the slot expression (see examples)
+    domain: class_definition
+    ifabsent: false
+    range: extra_slots_expression
+    in_subset:
+      - SpecificationSubset
+      - BasicSubset
+    examples:
+      - value:
+          allowed: true
+        description: Allow all additional data
+      - value:
+          allowed: false
+        description: Forbid any additional data
+      - value:
+          range_expression:
+            range: string
+        description: Allow additional data that are strings
+      - value:
+          range_expression:
+            range: AClassDefinition
+        description: Allow additional data if they are instances of the class definition "AClassDefinition"
+      - value:
+          range_expression:
+            any_of:
+              - range: string
+              - range: integer
+        description: allow additional data if they are either strings or integers
+      - value:
+          range_expression:
+            range: integer
+            multivalued: true
+            maximum_cardinality: 5
+        description: |
+          Allow additional data if they are lists of integers of at most length 5.
+          Note that this does *not* mean that a maximum of 5 extra slots are allowed.
+      - value:
+          range_expression:
+            range: integer
+            required: true
+        description: |
+          Allow additional data if they are integers. 
+          `required` is meaningless in this context and ignored, since by definition all "extra" slots are optional.
+      - value:
+          allowed: false
+          range_expression:
+            range: string
+        description: |
+          A semantically *invalid* use of `extra_slots`, as extra slots will be forbidden and the 
+          `anonymous_slot_expression` will be ignored.
+
+  allowed:
+    description: Whether or not something is allowed. Usage defined by context.
+    range: boolean
+    in_subset:
+      - SpecificationSubset
+      - BasicSubset
 
   # -----------------------------------
   # Slot definition slots
@@ -3023,6 +3089,7 @@ classes:
       - represents_relationship
       - disjoint_with
       - children_are_mutually_disjoint
+      - extra_slots
     slot_usage:
       is_a:
         range: class_definition
@@ -3226,6 +3293,18 @@ classes:
     in_subset:
       - SpecificationSubset
 
+  extra_slots_expression:
+    description: |
+      An expression that defines how to handle additional data in an instance of class
+      beyond the slots/attributes defined for that class. 
+      See `extra_slots` for usage examples.
+    mixins:
+      - expression
+    slots:
+      - allowed
+      - range_expression
+
+
 # ==================================
 # Enumerations                     #
 # ==================================