This is used as the interface between S7 and R's other OO systems, allowing
you to use S7 classes and methods with base types, informal S3 classes, and
formal S4 classes.
Usage
as_class(x, arg = deparse(substitute(x)))
Value
A standardised class: either NULL, an S7 class, an S7 union,
as new_S3_class, or a S4 class.