Go gotcha: What happened to ABBA?

The Trim, TrimLeft and TrimRight functions strip all Unicode code points contained in a cutset. In this case, all trailing A:s and B:s are stripped from the string, leaving the empty string.

To strip a trailing string, use strings.TrimSuffix.

fmt.Println(strings.TrimSuffix("ABBA", "BA"))
// Output: "AB"

See String handling cheat sheet for more about strings in Go.