docs: def merge(self, other):
          """Add requirements from 'other' Berksfile into this one."""
          if not isinstance(other, Berksfile):
              raise TypeError("Berksfile to merge should be a 'Berksfile' "
                              "instance, not %s.", type(other))
          current = self.to_dict()
          new = other.to_dict()
  
          # compare and gather cookbook dependencies
          berks_writelines = ['%s\n' % self.cookbook_statement(cbn, meta)
                              for cbn, meta in new.get('cookbook', {}).items()
                              if cbn not in current.get('cookbook', {})]
  
          # compare and gather 'source' requirements
          berks_writelines.extend(["source '%s'\n" % src for src
                                   in new.get('source', [])
                                   if src not in current.get('source', [])])
  
          self.write_statements(berks_writelines)
          return self.to_dict()