Delete items/keys from list/structs

[cueckoo]

Originally opened by @vnzongzna in cuelang/cue#519

Cue is perfect solution when we are thinking about the unification of items/structs. There are cases in our configuration when the end-user needs to delete items/keys from the existing list/struct like

DeleteThis: ["this", "that"]
useful_infra: {
	gcp: cluters:   10
	aws: clusters:  20
	this: clusters: 1
	that: clusters: 2
}

While one way to delete from useful_infra is

New_infra: {
    for k,v in useful_infra {
        if !list.Contains(DeleteThis, k) {
            "\(k)": v
        }
    }
}

Can we think of any other expression to do the disjunction as well?

[cueckoo]

Original reply by @mpvl in cuelang/cue#519 (comment)

Would a "cast" operator as described in Issue #454 work for you? Basically, instead of specifying what to remove, you would specify what to keep.