diff --git a/c/containers/mlr_dsl_ast.c b/c/containers/mlr_dsl_ast.c
index 527e71a59..cf166dab3 100644
--- a/c/containers/mlr_dsl_ast.c
+++ b/c/containers/mlr_dsl_ast.c
@@ -13,7 +13,6 @@ mlr_dsl_ast_node_t* mlr_dsl_ast_node_alloc(char* text, int type) {
 }
 
 // ----------------------------------------------------------------
-// xxx rename to ..._terminal to make it clear that children aren't copied.
 mlr_dsl_ast_node_t* mlr_dsl_ast_node_copy(mlr_dsl_ast_node_t* pother) {
 	mlr_dsl_ast_node_t* pnode = mlr_dsl_ast_node_alloc(pother->text, pother->type);
 	return pnode;
diff --git a/c/containers/percentile_keeper.c b/c/containers/percentile_keeper.c
index 3b25297e1..dfbf35088 100644
--- a/c/containers/percentile_keeper.c
+++ b/c/containers/percentile_keeper.c
@@ -53,10 +53,10 @@ static int compute_index(int n, double p) {
 		index = 0;
 	else if (index >= n)
 		index = n-1;
-	// xxx need to try harder on round-up/round-down cases?
 	return index;
 }
 
+// See also https://github.com/johnkerl/miller/issues/14 which requests an interpolation option.
 double percentile_keeper_emit(percentile_keeper_t* ppercentile_keeper, double percentile) {
 	if (!ppercentile_keeper->sorted) {
 		qsort(ppercentile_keeper->data, ppercentile_keeper->size, sizeof(double), double_comparator);
diff --git a/c/containers/top_keeper.c b/c/containers/top_keeper.c
index 2db8197a3..3090a3fa1 100644
--- a/c/containers/top_keeper.c
+++ b/c/containers/top_keeper.c
@@ -6,7 +6,6 @@
 // ----------------------------------------------------------------
 top_keeper_t* top_keeper_alloc(int capacity) {
 	top_keeper_t* ptop_keeper = mlr_malloc_or_die(sizeof(top_keeper_t));
-	// xxx mk func for neg-cap check; use here & elsewhere
 	ptop_keeper->top_values   = mlr_malloc_or_die(capacity*sizeof(double));
 	ptop_keeper->top_precords = mlr_malloc_or_die(capacity*sizeof(lrec_t*));
 	ptop_keeper->size         = 0;
diff --git a/c/mapping/mapper_having_fields.c b/c/mapping/mapper_having_fields.c
index 410768ef1..a3b49cf48 100644
--- a/c/mapping/mapper_having_fields.c
+++ b/c/mapping/mapper_having_fields.c
@@ -50,12 +50,12 @@ static sllv_t* mapper_having_fields_which_are_process(lrec_t* pinrec, context_t*
 // ----------------------------------------------------------------
 static sllv_t* mapper_having_fields_at_most_process(lrec_t* pinrec, context_t* pctx, void* pvstate) {
 	if (pinrec == NULL)
-		return sllv_single(NULL); // xxx cmt all of these, in all mappers
+		return sllv_single(NULL);
 	mapper_having_fields_state_t* pstate = (mapper_having_fields_state_t*)pvstate;
 	for (lrece_t* pe = pinrec->phead; pe != NULL; pe = pe->pnext) {
 		if (!hss_has(pstate->pfield_name_set, pe->key)) {
 			lrec_free(pinrec);
-			return NULL; // xxx cmt all of these, in all mappers
+			return NULL;
 		}
 	}
 	return sllv_single(pinrec);
diff --git a/c/mapping/mapper_put.c b/c/mapping/mapper_put.c
index 622a084b2..29349390d 100644
--- a/c/mapping/mapper_put.c
+++ b/c/mapping/mapper_put.c
@@ -40,7 +40,6 @@ static void mapper_put_free(void* pvstate) {
 	free(pstate->pevaluators);
 }
 
-// xxx comment me ...
 static mapper_t* mapper_put_alloc(sllv_t* pasts) {
 	mapper_put_state_t* pstate = mlr_malloc_or_die(sizeof(mapper_put_state_t));
 	pstate->num_evaluators = pasts->length;
diff --git a/c/mapping/mlr_val.c b/c/mapping/mlr_val.c
index 6b9a5f729..a39c8f1c0 100644
--- a/c/mapping/mlr_val.c
+++ b/c/mapping/mlr_val.c
@@ -82,7 +82,6 @@ int mt_get_boolean_strict(mv_t* pval) {
 }
 
 // ----------------------------------------------------------------
-// xxx check for semantics comparable to mt_get_boolean_strict
 void mt_get_double_strict(mv_t* pval) {
 	if (pval->type == MT_NULL)
 		return;
@@ -112,7 +111,6 @@ void mt_get_double_strict(mv_t* pval) {
 }
 
 // ----------------------------------------------------------------
-// xxx merge with mt_get_double_string w/ a nullable parameter
 void mt_get_double_nullable(mv_t* pval) {
 	if (pval->type == MT_NULL)
 		return;
@@ -192,7 +190,6 @@ mv_t s_sss_sub_func(mv_t* pval1, mv_t* pval2, mv_t* pval3) {
 }
 
 // ----------------------------------------------------------------
-// xxx cmt mem-mgt & contract. similar to lrec-mapper contract.
 mv_t s_s_tolower_func(mv_t* pval1) {
 	char* string = mlr_strdup_or_die(pval1->u.strv);
 	for (char* c = string; *c; c++)
@@ -204,7 +201,6 @@ mv_t s_s_tolower_func(mv_t* pval1) {
 	return rv;
 }
 
-// xxx cmt mem-mgt & contract. similar to lrec-mapper contract.
 mv_t s_s_toupper_func(mv_t* pval1) {
 	char* string = mlr_strdup_or_die(pval1->u.strv);
 	for (char* c = string; *c; c++)