check-record.mo:8.11-8.16: type error [M0149], expected mutable 'var' field v of type
  Nat
but found immutable field (insert 'var'?)
check-record.mo:10.11-10.20: type error [M0150], expected immutable field a of type
  Nat
but found mutable 'var' field (delete 'var'?)
check-record.mo:12.11-12.16: type error [M0149], expected mutable 'var' field a of type
  Int
but found immutable field (insert 'var'?)
check-record.mo:14.9-14.11: type error [M0151], object literal is missing field a from expected type
  {a : Nat}
check-record.mo:21.30-21.31: type error [M0057], unbound variable c
check-record.mo:24.9-24.27: type error [M0096], expression of type
  {a : Nat}
cannot produce expected type
  {b : Nat}