code: def appropriate_for(self, usage, alg='HS256'):
          
          try:
              _use = USE[usage]
          except:
              raise ValueError('Unknown key usage')
          else:
              if not self.use or self.use == _use:
                  if _use == 'sig':
                      return self.get_key()
                  else:
                      return self.encryption_key(alg)
  
              raise WrongUsage("This key can't be used for {}".format(usage))
docs: Make sure there is a key instance present that can be used for
          the specified usage.