how to specify string with elements in list?

[imacks]

I know I can constrain a parameter to one of items in a list:

/** @typedef {'a'|'b'|'c'} Alphabet */
/** @param {Alphabet} s */
function write(s) {
  // s will be one of 'a', 'b', or 'c'
}
write('a') // works
write('z') // fails

But what if it's a combination of items in list that is also a string?

write('a b') // works
write('c') // works
write('a z') // fails

I can constrain the input type to Array.<Alphabet> but I do need it to be a string. Help appreciated.