同时使用字符串和子字符串

鉴于:

julia> SubString <: String
false

如何编写同时接受子字符串和字符串的函数?

julia> function myfunction(ss::String)
           @show ss, typeof(ss)
       end
myfunction (generic function with 1 method)

julia> myfunction("Hello World")
(ss, typeof(ss)) = ("Hello World", String)
("Hello World", String)

julia> s = split("Hello World")
2-element Array{SubString{String},1}:
 "Hello"
 "World"

julia> foreach(x -> myfunction(x), s)
ERROR: MethodError: no method matching myfunction(::SubString{String})
Closest candidates are:
  myfunction(::String) at REPL[11]:2