Triplex arithmetic uses a similar string defintion as normal intervals, see 4.1, but extended to three values. These are the infinimum, value and supremum.
For a singleton triplex, where 
a single value is represented in brackets. Examples include [8.3], [8].
For normal triplexes, the infinimum, value and supremum are written in brackets, separated by a comma.
Examples include [8.3,10.1,12.2].
Empty intervals can be specified as both a singleton or a normal triplex with instead of numbers the string nan.
Examples include: [nan] and [nan,nan,nan].