The ta.subset function works when using a constant, but not when using an R object or an Aster R object. This example shows how to work around this limitation:
ta.dbx3 <- ta.dbx[ta.dbx$hdl > 0,] ta.dbx3 <- ta.dbx[ta.dbx$hdl > ta.dbx$ldl,] ta.dbx4 <- ta.dbx[ta.dbx$hdl == ta.dbx$ldl,]