docs: def unregister(cls, code):
          """Remove an application-specific function from the registry.
  
          Unregisters the function with the given `code` (an integer).  If the
          function is not registered, a `KeyError` is raised.  Unregistering a
          function with a `code` not in the application-specific range
          (0x00-0xff) raises a `ValueError`.
  
          >>> import hashlib
          >>> FuncReg.register(0x05, 'md-5', 'md5', hashlib.md5)
          >>> FuncReg.get('md-5')
          5
          >>> FuncReg.unregister(0x05)
          >>> FuncReg.get('md-5')
          Traceback (most recent call last):
              ...
          KeyError: ('unknown hash function', 'md-5')
          """
          if code in Func:
              raise ValueError(
                  "only application-specific functions can be unregistered")
          # Remove mapping to function by name.
          func_names = {n for (n, f) in cls._func_from_name.items() if f == code}
          for func_name in func_names:
              del cls._func_from_name[func_name]
          # Remove hashlib data and mapping to hash.
          hash = cls._func_hash.pop(code)
          if hash.name:
              del cls._func_from_hash[hash.name]