public ReturnType getReturnType() {
        return returnType;
    }
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