```ts
/**
 * Expands a span of the form "a-z" into all letters from a to z inclusive.
 */
export function spanLetters(span: string): string {
  if (span.length !== 3 || span[1] !== "-") {
    return span;
  }

  const from = span.charCodeAt(0);
  const to = span.charCodeAt(2);

  if (from > to) {
    return span;
  }

  let out = "";
  for (let code = from; code <= to; code++) {
    out += String.fromCharCode(code);
  }
  return out;
}
```