+ * hbytes mask-map lookup MAP-VAR ADDRESS [DEFAULT] => DATA
+ * error on missing default or if address too short
+ * hbytes mask-map amend MAP-VAR PREFIX PREFIX-LENGTH DATA
+ * a maskmap MAP is [list [list PREFIX PREFIX-LENGTH DATA]]
+ * sorted first by descending PREFIX-LENGTH and then by PREFIX
+ * each PREFIX is truncated to the shortest number of pairs of
+ * hex digits which can represent it
+ *