public final MultiArgFunctionNode addArg(ExpressionNode arg) {
        Objects.requireNonNull(arg, "Can not add null arg"); // throws NullPointerException
        args.add(arg);
        return this;
    }
<p>Adds the given argument to this function.</p>

@param arg The argument to add.
@return This, to allow chaining.
searchlib/src/main/java/com/yahoo/searchlib/expression/MultiArgFunctionNode.java