Conversely to C, Java, or many programming languages, this syntax IS NOT an array access syntax, but an iterative call that requires iteration copies of the template template-name. Each copy will be indepentendly processed by the processor and may return different content, especially when it contains matrix/vector calls or conditional statements.
This statement trigs the Automated Index Mechanism for any embedded vector/matrix call, thus allowing a very light-syntax multidimentional exploration. If an embeded vector/matrix call has no data left (i.e. indexes have flown over effective existing data array), it will return the standard default value of a scalar call.