S4.md

errors on non-S4 classes

Code
  S4_to_R7_class(1)
Error <simpleError>
  Unsupported S4 object: must be a class generator or a class definition, not a <double>.

S4 registration: checks its inputs

Code
  S4_register("x")
Error <simpleError>
  `class` must be an R7 class, not a <character>