join.go

type interState struct {
	Return string // final token value
	Save   string // running value
}
func isNumber(s String) bool { /* ... */ }
// Tokens observable filter operator ...
Scan(func(_ context.Context, acc interface{}, elem interface{}) (interface{}, error) {
  var state interState
  state, isToken := acc.(interState)
  state.Return = ""
  if !isToken || (isNumber(state.Save) && isNumber(elem.(string)) {
    state.Save += elem.(string)
  } else {
    state.Return = state.Save
    state.Save = elem.(string)
  }
  return state, nil
})