Amendments to the "new literal profile" PR (#387)

This commit amends the #384 PR to:

* describe what literal mappings are and how they should be used;
* remove the old page about “SSSOM profiles”, which is no longer
relevant (literal mappings are not a “profile”);
* attempt to better describe `rdfs literal` in the built-in schema
documentation.

mkdocs.yml

@@ -33,7 +33,6 @@ nav:
       - Introduction: spec-model.md
       - Applying Chaining Rules: chaining-rules.md
       - LinkML documentation: linkml-index.md
-      - SSSOM profiles: sssom-profiles.md
     - Serialisations:
       - Introduction: spec-formats.md
       - SSSOM/TSV serialisation: spec-formats-tsv.md

src/docs/spec-model.md

@@ -83,6 +83,25 @@ In addition, predicates from the following sources MAY also be encouraged:
 * any relation under [skos:mappingRelation](http://www.w3.org/2004/02/skos/core#mappingRelation) in the [Semantic Mapping Vocabulary](https://mapping-commons.github.io/semantic-mapping-vocabulary/).
 
 
+## Literal mappings
+
+<a id="literal-mappings"></a>
+
+The SSSOM model is primarily intended to represent mappings between semantic entities. However, it may also be used to represent mappings where one side is a literal string that does not have an identifier of its own. Any such mapping is henceforth called a _literal mapping_.
+
+To represent a mapping whose subject (resp. object) is a literal:
+
+* the `subject_type` (resp. `object_type`) slot is set to `rdfs literal`;
+* the `subject_label` (resp. `object_label`) slot is set to the literal itself;
+* the `subject_id` (resp. `object_id`) slot is left empty.
+
+The last point is an exception to the normal rules about required slots, which state that a mapping must always have a `subject_id` and an `object_id`. Implementations MUST accept a mapping without a `subject_id` (resp. `object_id`) _if and only if_ the `subject_type` (resp. `object_type`) slot is set to `rdfs literal`.
+
+All other slots in the `Mapping` class may be used normally in a literal mapping, with the same meaning as for a non-literal mapping.
+
+When computing the cardinality of mappings in a set (e.g. to set the value of the `mapping_cardinality` slot), if the mapping has a literal subject (resp. object), then the `subject_label` (resp. `object_label`) slot must be used for determining the number of occurrences of the subject (resp. object) in the set.
+
+
 ## Representing unmapped entities
 
 The special value `sssom:NoTermFound` MAY be used as the `object_id` of a mapping to explicitly state that the subject of said mapping cannot be mapped to any entity in the domain represented by the `object_source` slot.

src/sssom_schema/schema/sssom_schema.yaml

@@ -45,9 +45,9 @@ enums:
         meaning: rdfs:Class
       rdfs literal:
         meaning: rdfs:Literal
-        description: "'rdfs literal' used in SSSOM files to express that an entity is represented by a literal."
+        description: This value indicate that the entity being mapped is not a semantic entity with a distinct identifier, but is instead represented entirely by its literal label. This value MUST NOT be used in the predicate_type slot.
         see_also:
-        - https://mapping-commons.github.io/sssom/sssom-profiles/
+        - https://mapping-commons.github.io/sssom/spec-model/#literal-mappings
         - https://github.com/mapping-commons/sssom/issues/234
       rdfs datatype:
         meaning: rdfs:Datatype