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

S7_class validates its underlying data

<X> object is invalid:
- Underlying data is corrupt

$ gives useful error

Code
  x$y
Condition
  Error:
  ! Can't get S7 properties with `$`. Did you mean `x@y`?
Code
  x$y <- 1
Condition
  Error:
  ! Can't set S7 properties with `$`. Did you mean `...@y <- 1`?

[ gives more accurate error

Code
  x <- new_class("foo")()
  x[1]
Condition
  Error in `check_subsettable()`:
  ! S7 objects are not subsettable.
Code
  x[1] <- 1
Condition
  Error in `check_subsettable()`:
  ! S7 objects are not subsettable.

[[ gives more accurate error

Code
  x <- new_class("foo")()
  x[[1]]
Condition
  Error in `check_subsettable()`:
  ! S7 objects are not subsettable.
Code
  x[[1]] <- 1
Condition
  Error in `check_subsettable()`:
  ! S7 objects are not subsettable.