# expected data point 1 |--------------------------------| 154
#
ifl!=len(df['index_chain']):# if some residues are missing, len(df['index_chain']) < l
resnum_start=df.iloc[0,1]
diff=set(range(l)).difference(df['nt_resnum']-resnum_start)# the rowIDs the missing nucleotides would have (rowID = index_chain - 1 = nt_resnum - resnum_start)