class-spec.md
In S7: An Object Oriented System Meant to Become a Successor to S3 and S4

as_class gives informative errors

Code
  as_class("foo")
Condition
  Error:
  ! Can't convert `"foo"` to a valid class. Class specification must be an S7 class object, the result of `new_S3_class()`, an S4 class object, or a base class, not a <character>.
Code
  as_class(TRUE)
Condition
  Error:
  ! Can't convert `TRUE` to a valid class. Class specification must be an S7 class object, the result of `new_S3_class()`, an S4 class object, or a base class, not a <logical>.