public Optional<String> getRef() {
        return Optional.ofNullable(ref);
    }
Defines a single JSON schema.

<p>This supports a subset of the "draft-handrews-json-schema-validation-01"
version of JSON Schema. The following properties are not supported:

<ul>
    <li>dependencies</li>
    <li>if</li>
    <li>then</li>
    <li>else</li>
    <li>examples</li>
</ul>

<p>Custom properties can be added to the scheme using the
{@link Builder#putExtension} method. Custom properties are merged with
the schema document when serializing it to a {@link Node} object. Any
extension properties that conflict with built-in properties overwrite the
built-in property.

@see <a href="https://json-schema.org/latest/json-schema-validation.html">JSON Schema Validation</a>
smithy-jsonschema/src/main/java/software/amazon/smithy/jsonschema/Schema.java