Restrict a field to have only one level of nesting for an embedded array

one_level_nesting.ts

type UnionTy = 'a' | 'b';

type Arr = Array<UnionTy | UnionTy[]>;

interface Demo {
  prop: Arr,
}

const demo: Demo = {
  prop: ['a', ['a', 'b']]
  // prop: ['a', ['a', 'b', ['a']]] // => Errors.
};

console.log(demo);