alternating-case-equals-alternating-case

altERnaTIng cAsE <=> ALTerNAtiNG CaSe

Define String.prototype.toAlternatingCase (or a similar function/method such as to_alternating_case/toAlternatingCase/ToAlternatingCase in your selected language; see the initial solution for details) such that each lowercase letter becomes uppercase and each uppercase letter becomes lowercase. For example:

StringUtils.toAlternativeString("hello world") == "HELLO WORLD"
StringUtils.toAlternativeString("HELLO WORLD") == "hello world"
StringUtils.toAlternativeString("hello WORLD") == "HELLO world"
StringUtils.toAlternativeString("HeLLo WoRLD") == "hEllO wOrld"
StringUtils.toAlternativeString("12345") == "12345" // Non-alphabetical characters are unaffected
StringUtils.toAlternativeString("1a2b3c4d5e") == "1A2B3C4D5E"
StringUtils.toAlternativeString("StringUtils.toAlternatingCase") == "sTRINGuTILS.TOaLTERNATINGcASE"

As usual, your function/method should be pure, i.e. it should not mutate the original string.