Selecting first non-null
Choosing the first non-null item in a sequence
For example, in IATI data, the currency for an item can be set at the level of a value in a transaction, the transaction or the overall activity, and if not set is USD.
since nulls are ignored in a sequence, values can be put in a sequence ordered from highest to lowest priority from which we extract the first element.
Nested if .. then .. else statements