+ * 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 ascending PREFIX
+ * each PREFIX _in the map_ is truncated to the shortest number of
+ * pairs of hex digits which can represent it
+ *