"ABC" : string
"abcBAD" : string
"abcA" : list char
"AB" : list char
"ABC" : list char
"abcD" : list char
append "abc" "cde" : list char
str 67 (str 66 (str 65 empty)) : string
str 68 (str 65 (str 66 (str 99 (str 98 (str 97 empty))))) : string
65 :: str 99 (str 98 (str 97 empty)) : list char
[66, 65] : list char
[67, 66, 65] : list char
68 :: str 99 (str 98 (str 97 empty)) : list char
append (str 99 (str 98 (str 97 empty))) (str 101 (str 100 (str 99 empty))) : list char