code: def returns(returntype):
      
      returntype = T.TypeFactory(returntype)
      def _decorator(func):
          # @returns decorator
          if U.has_fun_prop(func, "returntype"):
              raise ValueError("Cannot set return type twice")
          U.set_fun_prop(func, "returntype", returntype)
          return _wrap(func)
      return _decorator
docs: A function decorator to specify return type of the function.
  
      Example usage:
  
        | @accepts(Positive0)
        | @returns(Positive0)
        | def square_root(x):
        |     ...