How to flatten an array of arrays of objects using Groq in Sanity.io.

backgrounds: [
  [{ asset: {...} }],
  [{ asset: {...} }]
]

thumbnails: [
  { asset: {...} },
  { asset: {...} }
],

'thumbnails': ...backgrounds[]->{
  backgrounds[]{asset}
}
// Unexpected prefix operator "dotDotDot", which makes sense

'thumbnails': [...backgrounds[]->{
  backgrounds[]{asset}
}]
// Nothing flattened

'thumbnails': [...backgrounds[]->{
  ...backgrounds[]{asset}
}]
// thumbnails: [ {}, {} ]

'thumbnails': [...backgrounds[]->{
  backgrounds[]{asset}
}.backgrounds]
// Nothing flattened

export default {
  title: 'Background list',
  name: 'backgrounds',
  type: 'document',
  fields: [
    {
      localize: false,
      title: 'Name',
      description: 'A short name describing what this list of backgrounds contains.',
      name: 'text',
      type: 'string',
    },
    {
      title: 'Backgrounds',
      name: 'backgrounds',
      type: 'array',
      of: [{ type: 'image', options: { accept: 'image/*' } }],
    },
  ],
};

'backgroundLists': backgroundLists[].asset->,

"backgrounds": [
  null,
  null
]

'backgroundLists': backgroundLists[]->.backgrounds[].asset,