ExpressionFunction requireReferencedFunction(FunctionReference reference) {
        ExpressionFunction function = referencedFunctions.get(reference);
        if (function == null)
            throw new IllegalArgumentException("No " + reference + " in " + this + ". References: " +
                                               referencedFunctions.keySet().stream()
                                                                           .map(FunctionReference::serialForm)
                                                                           .collect(Collectors.joining(", ")));
        return function;
    }
Returns the given referred function, or throws a IllegalArgumentException if it does not exist
model-evaluation/src/main/java/ai/vespa/models/evaluation/Model.java