transferring selected items in an interface to a rule input where it can be transferred to a process model

ok, so I've got an interface which is a  process start form for my process model.

In my interface I've got a gridfield in which I display a list of cases, and this gridfield is configured to allow me to select the ones that I want to assign to someone.

What I want to do is [having selected some of the cases with checkboxes] then populate a rule input with just those selected cases so I can then pass that into my process model for reassignment.

The gridfield looks like this:

label: "Cases",
labelPosition: "ABOVE",
totalCount: count(
local!dbCases.data
),
columns: {
a!gridTextColumn(
label: "Case Number",
field: "caseId",
data: index(
local!dbCases.data,
"caseId"
)
),
a!gridTextColumn(
label: "Case Project Name",
field: "caseProjectName",
data: index(
local!dbCases.data,
"caseProjectName"
)
),
a!gridTextColumn(
label: "Case Owner",
field: "caseOwnerName",
data: index(
local!dbCases.data,
"caseOwnerName"
)
)
},
identifiers: local!dbCases.identifiers,
value: local!dataGridSelection,
saveInto: {
local!dataGridSelection
},
selection: true,
validations: {},
shadeAlternateRows: true
)

And the gridSelection looks like this:

local!dataGridSelection: a!gridSelection(
selected: {},
pagingInfo: a!pagingInfo(
startIndex: 1,
batchSize: count(
local!dbCases.data
),
sort: a!sortInfo(
field: "caseId",
ascending: true
)
)
)

The local!dbCases local variable is a the returned value of a a!queryEntity() call which pulls back my cases...

So - I *think* my local variable local!dataGridSelection will now have the selected items [but it's hard to tell without any kind of debugger tool (a source of substantial frustration)]

What I want to know though - is how do I get [only] the selected items into a rule input that I can then use in the rest of my process model?

Here's the code for my "assignment" button:

a!buttonArrayLayout(
buttons: {
a!buttonWidget(
label: "Transfer",
saveInto: {
a!forEach(    /* <--- do I use some kind of forEach to loop through the grid selection, and how do I figure out which ones are selected?  */
local!dataGridSelection,
a!save(
target: null,      /*  <----- what do I put here?  */
value: a!gridSelection(local!dataGridSelection)  /* <--- also, I'm pretty sure this isn't right */
)
)
},
style: "NORMAL"
)
},
align: "START"
)

Any help much appreciated.

thanks heaps,

David.