How can I layout structures like this?

  struct field
    {
     unsigned short type;
     void *value;
    };

  struct multifield
    {
     struct field theFields[1];
    };

If theFields was another type, say a pointer, then I'd try something
like this (although I'm not sure if this would be correct either):

      class Field < FFI::Struct
        layout :type, :ushort, :value, :pointer
      end
      class Multifield < FFI::Struct
        layout :theFields, [:pointer, 1]
      end

How can I specify 'struct field' as a type?