"A singleton [[Tuple]] with the given [[element]] if the 
 given element is non-null, or the [[empty sequence|Empty]] 
 otherwise. This operation transforms an optional type `T?`
 to a sequence type `[]|[T]` allowing optional values to be
 the subject of operations defined for [[streams|Iterable]].
 For example, [[flat mapping|Iterable.flatMap]] 
 `emptyOrSingleton()` reproduces the behavior of 
 [[Iterable.coalesced]]. The expression
     { \"1.23\", \"foo\", \"5.67\", \"-1\", \"\" }
 produces the stream:
     { 1.23, 5.67, -1.0 }"
see (`class Tuple`, `interface Empty`)
shared []|[Element&Object] emptyOrSingleton<Element>
        (Element element)
        => if (exists element) then [element] else [];