eclipse-atl - “at”函数不适用于有序集

Question

使用下面的代码确保有序集不为空，但会出现以下错误

(urn!PathNode.allInstances()).at(1).debug();

org.eclipse.m2m.atl.engine.emfvm.VMException: Operation not found: OrderedSet {OUT!<unnamed>, OUT!<unnamed>, OUT!<unnamed>, OUT!<unnamed>, OUT!<unnamed>, OUT!<unnamed>, OUT!<unnamed>, OUT!<unnamed>, OUT!<unnamed>, OUT!<unnamed>}.at(java.lang.Integer)