``` ucm :hide
proj/main> builtins.merge lib.builtin
```

``` unison
lib.old.foo = 17
lib.new.foo = +18
thingy = lib.old.foo + 10
```

``` ucm
proj/main> add
```

``` ucm :error
proj/main> upgrade old new
```

Resolve the error and commit the upgrade.

``` unison
thingy = foo + +10
```

``` ucm
proj/upgrade-old-to-new> update
proj/upgrade-old-to-new> upgrade.commit
proj/main> view thingy
proj/main> ls lib
proj/main> branches
```