setLit.lean:22:19-22:21: error: overloaded, errors 
  failed to synthesize
    EmptyCollection String
  Additional diagnostic information may be available using the `set_option diagnostics true` command.
  
  fields missing: 'data'
setLit.lean:24:31-24:38: error: overloaded, errors 
  failed to synthesize
    Singleton Nat String
  Additional diagnostic information may be available using the `set_option diagnostics true` command.
  
  24:33 'val' is not a field of structure 'String'