# File lib/specinfra/command/nixos/base/package.rb, line 3
    def check_is_installed(package, version=nil)
      if version
        "nix-store -q --references /var/run/current-system/sw | grep #{escape(package)}-#{escape(version)}"
      else
        "nix-store -q --references /var/run/current-system/sw | grep #{escape(package)}"
      end
    end