[phenixbb] on phenix.real_space_refine

Dear All, If I only want to refine resi 30-50 in Chain A by phenix.real_space_refine, and my protein contains A, B, C chains of a identical sequence of 800 aas, will you please let me know how can I specify "residue 30-50 of Chain A" in the phenix.real_space_refine command? Smith