def fetch(fn)
      return fn unless fn.instance_of? Symbol
      respond_to?(fn) ? method(fn) : store.fetch(fn)
    rescue
      raise FunctionNotFoundError.new(fn, self)
    end
def fetch(fn)
      return fn unless fn.instance_of? Symbol
      respond_to?(fn) ? method(fn) : store.fetch(fn)
    rescue
      raise FunctionNotFoundError.new(fn, self)
    end
Gets the procedure for creating a transproc

 @param [#call, Symbol] fn
   Either the procedure, or the name of the method of the current module,
   or the registered key of imported procedure in a store.

 @return [#call]