I have an example dataset, with 10 variables. Among them, i have two nominal variables, 'cluster' and 'cluster_from_ES2', both taking the values, 'cluster_0', 'cluster_1' and 'cluster_2'. Now, i want to delete the examples (rows), where cluster!=cluster_from_ES2. i.e., I want to retain only the rows when the value of 'cluster' is equal to 'cluster_from_ES2'. How do i do this?