tbl() is the standard constructor for tbls. as.tbl() coerces,
and is.tbl() tests.
make_tbl(subclass, ...)name of subclass. "tbl" is an abstract base class, so you
must supply this value. tbl_ is automatically prepended to the
class name
For tbl(), other fields used by class. For as.tbl(),
other arguments passed to methods.
to test/coerce.