parseExpression(e, t) {
    for (var r = []; ; ) {
      this.mode === "math" && this.consumeSpaces();
      var a = this.fetch();
      if (ts.endOfExpression.indexOf(a.text) !== -1 || t && a.text === t || e && an[a.text] && an[a.text].infix)
        break;
      var i = this.parseAtom(t);
      if (i) {
        if (i.type === "internal")
          continue;
      } else break;
      r.push(i);
    }
    return this.mode === "text" && this.formLigatures(r), this.handleInfixNodes(r);
  }
Parses a group with optional super/subscripts.