anchor.diff:      List = [ {pubkey,     C#contract.pubkey}
              , {balance,    C#contract.balance}
              , {height,     C#contract.height}
  +           , {nonce,      C#contract.nonce}
              , {owner,      C#contract.owner}
              , {vm_version, C#contract.vm_version}
              , {code,       C#contract.code}
  
positive.code: -spec set_height(height(), contract()) -> contract().
  set_height(X, C) ->
      C#contract{height = assert_field(height, X)}.
  
  -spec set_owner(pubkey(), contract()) -> contract().
  set_owner(X, C) ->
  
negative.code:         pubkey     :: pubkey(),
          balance    :: amount(),
          height     :: height(),
          nonce      :: non_neg_integer(),
          %% Contract-specific fields
          owner      :: pubkey(),
          vm_version :: vm_version(),
  