docs: def format_struct(struct_def):
      '''Returns a cython struct from a :attr:`StructSpec` instance.
      '''
      text = []
      text.append('cdef struct {}:'.format(struct_def.tp_name))
      text.extend(
          ['{}{}'.format(tab, format_variable(var))
           for var in struct_def.members]
      )
  
      for name in struct_def.names:
          text.append('ctypedef {} {}'.format(struct_def.tp_name, name))
  
      return text