Currently, nest can be used to create an entity with a single containment navigation property holding the result of a transformation sequence:

nest( cnp, t ) returns

[{ <cnp>: [ <result of t applied to input set> ] }]

There are use cases to generalize this and nest the results of multiple transformation sequences at once. Consider, for example, a request to combine the results of two transformations in a single response. Using transformation concat requires an extra computation of a concat index to relate an entity in the result to the corresponding concat argument (see ODATA-1138):

GET ~/Sales?$apply=concat(
   filter(<complex expression 1>)
   }}{{/groupby((Customer),aggregate(Amount with sum as Total))/compute(1 as concat_index),
   filter(<complex expression 2>)
   /groupby((Customer),aggregate(Amount with sum as Total))/compute(2 as concat_index))

If it makes sense to keep the results of the two transformations separately, they could be retrieved via an extended syntax of the transformation nest:

GET ~/Sales?$apply=nest(
   filter(<complex expression 1>)
   /groupby((Customer),aggregate(Amount with sum as Total)) as part_1,
   filter(<complex expression 2>)
   /groupby((Customer),aggregate(Amount with sum as Total)) as part_2)