Ah good point, two potential solutions:

1.  We coerce everything to tuples on input and so don't worry about hashability. 
2.  The type of the elements of the Python sequence ideally should be inferable from the TableExpr.  We don't do this well yet.  This is the difference between Pandas' `DataFrame` and `Series`.

Probably neither of these will happen now, so I now agree that your "just change the example" solution is best.