Clever workaround!
Here's another way I've used that may be more general and not requiring a formula (but there "should" be a simpler way..):
dt = Open( "$SAMPLE_DATA/Big Class.JMP" );
labels = {"a", "b", "c"}; // could be any sequence
dt << New Column( "Char Sequence2",
Character,
Nominal,
Values(
Insert(
Repeat( labels, N Rows( dt ) / N Items( labels ) ),
If( Mod( N Rows( dt ), N Items( labels ) ),
labels[1 :: Mod( N Rows( dt ), N Items( labels ) )],
{}
)
)
)
);