just comments

lib/src/estimate.c

@@ -4213,8 +4213,12 @@ MODEL arma (const int *list, const int *pqlags,
 
     gretl_model_init(&armod, dset);
 
+    /* --opg and --hessian cannot be combined */
     err = incompatible_options(opt, OPT_G | OPT_H);
     if (!err) {
+        /* --x-13arima cannot be combined with --robust, --stdx or
+           --lagselect. (Note that --stdx is not documented.)
+        */
 	err = options_incompatible_with(opt, OPT_X, OPT_R | OPT_S | OPT_Z);
     }
     if (err) {

plugin/arma.c

@@ -1243,11 +1243,12 @@ static int set_common_sample (arma_sel *asel, MODEL *amod,
 
 /* Provides built-in means of selecting the AR and MA orders of
    an AR(I)MA model via Information Criteria. We accept a general
+   specification of the form
 
    (p d q)(P D Q)
 
-   specification and search over p, q, P and Q, taking their values
-   on input as maxima; d and D remain clamped at their input values.
+   and search over p, q, P and Q, taking their values on input as
+   maxima; d and D remain clamped at their input values.
 */
 
 static int real_arma_select (const int *list, DATASET *dset,