private LazyArrayContext requireContextPrototype(String name) {
        LazyArrayContext context = contextPrototypes.get(name);
        if (context == null) // Implies function is not present
            throw new IllegalArgumentException("No function named '" + name + "' in " + this + ". Available functions: " +
                                               functions.stream().map(ExpressionFunction::getName).collect(Collectors.joining(", ")));
        return context;
    }
Returns the given function, or throws a IllegalArgumentException if it does not exist
model-evaluation/src/main/java/ai/vespa/models/evaluation/Model.java