docs: def make_structure(AA_chain,phi,psi_im1,omega=[]):
      '''Place a sequence of amino acids into a peptide with specified
      backbone dihedral angles. The argument AA_chain holds the
      sequence of amino acids to be used. The arguments phi and psi_im1 hold
      lists of backbone angles, one for each amino acid, *starting from
      the second amino acid in the chain*. The argument 
      omega (optional) holds a list of omega angles, also starting from
      the second amino acid in the chain.'''
      geo = geometry(AA_chain[0])
      struc=initialize_res(geo)
  
      if len(omega)==0:
          for i in range(1,len(AA_chain)): 
              AA = AA_chain[i]
              add_residue(struc, AA, phi[i-1], psi_im1[i-1])
      else:
          for i in range(1,len(AA_chain)): 
              AA = AA_chain[i]
              add_residue(struc, AA, phi[i-1], psi_im1[i-1], omega[i-1])
  
      return struc