if policy_session[:modalities] #array exists just add new value to it
    policy_session[:modalities] << [params[:modality], policy_session[:mode_list]]
else #the array does't exist yet, so create and add first one. 
    policy_session[:modalities] = [params[:modality], policy_session[:mode_list]]
	
>> policy_session[:modalities]
>># [["var_1"], "1",[["var_2"], ["2"]], [["var_3"], ["1"]]]
	
>>policy_session[:modalities]
>># [["var_1", "1"], ["var_2", "2"], ["var_3", "1"]]
	
policy_session[:modalities] ||= [] # set it to an empty array if nil
policy_session[:modalities] << [params[:modality], policy_session[:mode_list]]
	
policy_session[:modalities] ||= [] # set it to an empty array if nil
policy_session[:modalities] << [params[:modality], policy_session[:mode_list]].flatten