// handles only one repeat section so far
// returns pre, repeat section, post
static T3S tok_parseRepeatSymbols1(LS tok) {
  IntRange r1 = jfind_range(tok, "||:"), r2 = jfind_range(tok, ":||");
  if (r1 == null && r2 == null) null;
  if (r1 == null) r1 = IntRange(0, 0);
  if (r2 == null) r2 = IntRange(l(tok), l(tok));
  ret t3(joinSubList(tok, 1, r1.start),
    joinSubList(tok, r1.end, r2.start),
    joinSubList(tok, r2.end, l(tok)-1));
}