t- and th-, when they are the second letter, become dh-
function move_aspirate_forward(hash) {
    var asp = u.unasp2asp(hash.stemUlt);
    if (!asp) return;
    var stem = u.replaceEnd(hash.stem, hash.stemUlt, asp);
    if (!stem || stem == hash.stem) return;
    hash.stems = [stem];
    if (debug) log('mod: move_aspirate_forward', stem);
}