{d:nat}
?record binheap(d) {
  degree: int(d);
  key: int;
  parent: [pd: int | pd > d] binheap(pd);
  child: [cd: int] | cd < d] binheap(cd);
  sibling: binheap(d-1);
} with null: {d:int} binhead(d)

?record <'a> list with nat {
  head: 'a;
  tail: 'a * <'a> list(n)
} with null: list(0)