code: public Env<A> dupto(Env<A> that) {
          that.next = this;
          that.outer = this.outer;
          that.toplevel = this.toplevel;
          that.enclClass = this.enclClass;
          that.enclMethod = this.enclMethod;
          return that;
      }
docs: Duplicate this environment into a given Environment,
  using its tree and info, and copying all other fields.