Beginner question - how to efficiently check if a photo is part of the selection

Question

Dear all,
Sorry for this beginner question, however I don't know how to solve it and I'm sure that there is a quick and easy answer to this:
My target is to get all the "heads of stack" (photos with Position 1 in stacks) of the selection ("photos" below).
for i, photo in ipairs( photos ) do
  local photoName = photo:getFormattedMetadata('fileName')
   if photo:getRawMetadata( 'isInStackInFolder' ) then
   --Photo is in a stack
        local photoHeadOfStack = photo:getRawMetadata('topOfStackInFolderContainingPhoto')
        local stackPos = photo:getRawMetadata( 'stackPositionInFolder' ) -- get the position in the stack
        local photoHeadName = photoHeadOfStack:getFormattedMetadata('fileName')
        app:logVerbose("Photo nb ^1, name ^2 is in a stack with the position ^3", i, photoName, stackPos)
        if stackPos == 1 then
        -- photo is master of the stack
             stackTop[#stackTop+1]=photo
             app:logVerbose("Photo ^1 is top of the stack ^2", photoName, #stackTop)
             -- check if couple if correct
        elseif photos.photoHeadOfStack then
             app:logVerbose("Photo ^1 is in a stack, but not on top. Top of the stack already selected", photoName)
        else
             stackTop[#stackTop+1]=photoHeadOfStack
             app:logVerbose("Photo ^1 is in a stack, but not on top. Top of the stack ^2 - ^3 ", photoName, #stackTop, photoHeadName)
        end
  end
end
In the if structure in the middle checking the Position in the stack, if the Position is not 1, I would like to check if the photo heading the stack ('photoHeadOfStack') is part of 'photos'. I tried 'elseif photos.photoHeadOfStack then' and 'elseif photos[photoHeadOfStack] then' but it doesn't work ->this elseif is never true even if 'photoHeadOfStack' is part of 'photos'.
Is there anyway to have this check done without checking "manually" photos ?(via for example a for Loop)
I'm sure that Lua proposes this, but I don't know how...
I thank you for your Support!

Answer 1

    --[[ This code creates "stackTops", an array of LrPhotos that are the
    tops of all the stacks containing photos in the array
    "photos". ]]
stackTopSet = {}
for _, photo in ipairs (photos) do
    local stackTop = photo:getRawMetadata ("topOfStackInFolderContainingPhoto")
    if stackTop ~= nil then stackTopSet [stackTop] = true end
    end
stackTops = {}
for photo, _ in pairs (stackTopSet) do table.insert (stackTops, photo) end