code: def filesfile_string(self):
          
          lines = []
          app = lines.append
          pj = os.path.join
  
          app(self.input_file.path)                 # Path to the input file
          app(self.output_file.path)                # Path to the output file
          app(pj(self.workdir, self.prefix.idata))  # Prefix for input data
          app(pj(self.workdir, self.prefix.odata))  # Prefix for output data
          app(pj(self.workdir, self.prefix.tdata))  # Prefix for temporary data
  
          # Paths to the pseudopotential files.
          # Note that here the pseudos **must** be sorted according to znucl.
          # Here we reorder the pseudos if the order is wrong.
          ord_pseudos = []
  
          znucl = [specie.number for specie in
                   self.input.structure.types_of_specie]
  
          for z in znucl:
              for p in self.pseudos:
                  if p.Z == z:
                      ord_pseudos.append(p)
                      break
              else:
                  raise ValueError("Cannot find pseudo with znucl %s in pseudos:\n%s" % (z, self.pseudos))
  
          for pseudo in ord_pseudos:
              app(pseudo.path)
  
          return "\n".join(lines)
docs: String with the list of files and prefixes needed to execute ABINIT.