参见https://github.com/biopython/biopython/blob/master/Bio/PDB/Superimposer.py#L43:
else if (mol_opt=="RNA" && atom_opt=="auto") atom_opt=" C3'";if (u_opt && Lnorm_ass<=0) PrintErrorAndQuit("Wrong value for option -u! It should be >0"); if (d_opt && d0_scale<=0) PrintErrorAndQuit("Wrong value for option -d! It should be >0"); ...