Filters manipulate sets of titles in which no title may appear more than once. Furthermore, they often need to append one such set to another.

This is done in such a way that, if a title would be duplicated, the earlier copy of that title is discarded. The titles being appended are dominant.

For example, if a selection contains Andrew Becky Clara Daniel and Andrew Barney Clara Daisy is then appended to it, the result is Becky Daniel Andrew Barney Clara Daisy.

This behaviour can cause unexpected results when working with Mathematics Operators. For example, 1 2 3 +[sum[]] evaluates to 6, as expected. But 1 1 1 +[sum[]] evaluates to 1. Removing the +[sum[]] from each filter reveals the problem: 1 2 3 evaluates to the list 1, 2, 3, while 1 1 1 evaluates to the single item 1 due to de-duplication.

In such situations, the = prefix can be used to disable the de-duplication. For example, =1 =1 =1 +[sum[]] evaluates to 3 as expected. Alternatively, the split Operator can be used: [[1,1,1]split[,]sum[]].