Lifting: wrap a value into a tree so that we can reuse tree evaluation logic for partially evaluated trees, e.g. in https://github.com/scalareflect/interpreter/pull/8/files#diff-3fb462887cf14bd0ff5df477a1c37f65R207

Unlifting: extraction of wrapped values when evaluation reaches them: https://github.com/scalareflect/interpreter/pull/8/files#diff-3fb462887cf14bd0ff5df477a1c37f65R24