parse() {
    this.settings.globalGroup || this.gullet.beginGroup(), this.settings.colorIsTextColor && this.gullet.macros.set("\\color", "\\textcolor");
    try {
      var e = this.parseExpression(!1);
      return this.expect("EOF"), this.settings.globalGroup || this.gullet.endGroup(), e;
    } finally {
      this.gullet.endGroups();
    }
  }
Handle a subscript or superscript with nice errors.