Selection additions/Multiple active selections

### The Problem

There are a few instances in which selecting a new cuboid/cyl/sphere that I would like to also add another selection to what's currently selected. For instance, using //chunk to select multiple chunks for regeneration not in a simple cuboid or cutting/copying a very specific selection without the need to carve it out or mask its voids with air.

### A Solution

I believe the simplest solution would be a flag to choose whether to begin new selection or add to existing, where the former is the default and what we're all accustomed to, and the latter, something like the command "//sel addition" saves each selection to the 'master' selection in discrete additions. Alternatively, to enable editing of each selection, the command could add new position arguements of the form [(x1,y1,z1),(x2,y2,z2),(xn,yn,zn)] such that each pair/triplet/etc represents its own selection.

Something like this, where I would like to extract the green concrete without disturbing the others and then be able to place it thusly.

### Alternatives

The solution is simple if I have only the green concrete, select a 9x9, and "//paste -a", but becomes complex if the structure cannot be isolated with air first. Further, air masking with the -a option does not let one operate other methods on what would be the complex selection the structure represents (//regen, //set, //replace, etc)

### Anything Else?

Here I am "quarrying" multiple chunks and would like to regenerate certain chunks while leaving others bare, without the need to do each operation in sequence (e.g. "//chunk, //regen, //chunk, //regen ... ").

It would be easier to fly through and select each chunk, and then perform a //regen on all of them.