code: def get_ligands(pdb_id):
      
      out = get_info(pdb_id, url_root = 'http://www.rcsb.org/pdb/rest/ligandInfo?structureId=')
      out = to_dict(out)
      return remove_at_sign(out['structureId'])
docs: Return ligands of given PDB ID
  
      Parameters
      ----------
  
      pdb_id : string
          A 4 character string giving a pdb entry of interest
  
      Returns
      -------
  
      out : dict
          A dictionary containing a list of ligands associated with the entry
  
  
      Examples
      --------
      >>> ligand_dict = get_ligands('100D')
      >>> print(ligand_dict)
      {'id': '100D',
      'ligandInfo': {'ligand': {'@chemicalID': 'SPM',
                             '@molecularWeight': '202.34',
                             '@structureId': '100D',
                             '@type': 'non-polymer',
                             'InChI': 'InChI=1S/C10H26N4/c11-5-3-9-13-7-1-2-8-14-10-4-6-12/h13-14H,1-12H2',
                             'InChIKey': 'PFNFFQXMRSDOHW-UHFFFAOYSA-N',
                             'chemicalName': 'SPERMINE',
                             'formula': 'C10 H26 N4',
                             'smiles': 'C(CCNCCCN)CNCCCN'}}}