public SchemaNode getSymbolNode() {
        if (type == RankNodeType.EXPRESSION  || schemaNode.size() == 0) {
            return null;
        }

        Node symbolNode = schemaNode.get(0);
        if (!symbolNode.hasSymbol()) {
            return null;
        }

        return symbolNode.getSchemaNode();
    }
RankNode represents a node in the Rank pexression AST.
The node can either represent an expression, a feature or a built-in-function

An expression is a rank expression.
A feature is a function with parameters, and an optional property. All parameters must be expressions.
integration/schema-language-server/language-server/src/main/java/ai/vespa/schemals/tree/rankingexpression/RankNode.java