docs: def make_extended_structure(AA_chain):
      '''Place a sequence of amino acids into a peptide in the extended
      conformation. The argument AA_chain holds the sequence of amino
      acids to be used.'''
      geo = geometry(AA_chain[0])
      struc=initialize_res(geo)
      
      for i in range(1,len(AA_chain)): 
          AA = AA_chain[i]
          geo = geometry(AA)
          add_residue(struc, geo)
  
      return struc