I'm looking for a way to reference an element within a `List` to get another element in the same parent `List`. I can't really see any way to do this without either being able to specify a level that is *above* that element in a getter function or without some kind of self-referencing construct like `.this()` for object-oriented ABDs.

As a minimal example, is there a way to get

    listex={MemoryInUse[],f[x],MemoryInUse[],difference}

where `difference=listex[[3]]-listex[[1]]`, or is my sole option to generate a `List` without these dependent elements and then `Insert`/`Append` them post-creation?