Many of the interval arithmetic functions in the assertions are defined with argument lists that take 1 or more items. An example being the iaf:numeric-equal function : This describes a function that has two arguments, each of 1 or more items. If complete data is not provided, variables can provide a 'fallback' value as a default for missing items. In many cases these missing items do not always represent a single fact but a list of facts (sequence). For single numeric / monetary items, the fallback value is usually defined as zero : But for lists (sequences) the fallback value is usually defined to be an empty sequence : The empty sequence neither evaluates to zero, nor contains any items. As such, passing an empty sequence into a function that expects one or more items will cause a run-time XPath error. The XBRL formula specification provides no defined behaviour that an XBRL processor should implement when encountering such an error. Hard failure, some form of soft failure or attempted recovery or continuation are all possibilities. Moreover it is conceivable that some of these cases are actually valid, or at least not inconsistent, and therefore not errors. To avoid ending up in this situation, the assertions should be tolerant of these cases. Ensuriing that fallback values for sequences always produce a value would be one mechanism. Specifying '(0)' instead of '()' would achieve this and appear to be consistent from the point of view of the assertion. Another alternative would be to change the interval functions to allow inputs of zero or more items 'item()*' instead of one or more items. Some functions do already cater for empty lists through specific checks to handle empty input sequences. --- David Bell dbell@edgar-online.com +44 7876 396719