expandTokens(e) {
    var t = [], r = this.stack.length;
    for (this.pushTokens(e); this.stack.length > r; )
      if (this.expandOnce(!0) === !1) {
        var a = this.stack.pop();
        a.treatAsRelax && (a.noexpand = !1, a.treatAsRelax = !1), t.push(a);
      }
    return this.countExpansion(t.length), t;
  }
Determine whether a command is currently "defined" (has some
functionality), meaning that it's a macro (in the current group),
a function, a symbol, or one of the special commands listed in
`implicitCommands`.