+ * hbytes mask-map lookup MAP-VAR ADDRESS [DEFAULT] => DATA
+ * error on missing default or if any prefix longer than address
+ * 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
+ *
+ * Error codes
+ *
+ * HBYTES BLOCKCIPHER CRYPTFAIL CRYPT block cipher mode failed somehow (!)
+ * HBYTES BLOCKCIPHER CRYPTFAIL MAC HMAC failed somehow (!)
+ * HBYTES BLOCKCIPHER LENGTH block cipher input has unsuitable length
+ * HBYTES BLOCKCIPHER PARAMS key or iv not suitable
+ * HBYTES HMAC PARAMS key, input or output size not suitable
+ * HBYTES LENGTH OVERRUN block too long
+ * HBYTES LENGTH RANGE input length or offset is -ve or silly
+ * HBYTES LENGTH UNDERRUN block too short (or offset too big)
+ * HBYTES LENGTH MISMATCH when blocks must be exactly same length
+ * HBYTES SYNTAX supposed hex block had wrong syntax
+ * HBYTES VALUE OVERFLOW value to be conv'd to hex too big/long
+ * HBYTES MASKMAP NOMATCH no addr/mask matches address for lookup
+ * HBYTES MASKMAP UNDERRUN addr value for lookup is too short
+ * HBYTES MASKMAP SYNTAX LLENGTH value for mask-map entry not llength==3
+ * HBYTES MASKMAP SYNTAX UNDERRUN value for mask-map entry hex too short
+ * HBYTES MASKMAP SYNTAX OVERRUN actual mask-map entry hex too short
+ * HBYTES MASKMAP SYNTAX ORDER actual mask-map is disordered
+ * HBYTES MASKMAP SYNTAX EXCLBITS mask-map has 1-bits outside prefix len
+ * SOCKADDR AFUNIX LENGTH path for AF_UNIX socket too long
+ * SOCKADDR SYNTAX IPV4 bad IPv4 socket address &/or port
+ * SOCKADDR SYNTAX OTHER bad socket addr, couldn't tell what kind
+ * ULONG BITCOUNT NEGATIVE -ve bitcount specified where not allowed
+ * ULONG BITCOUNT OVERRUN attempt to use more than 32 bits
+ * ULONG BITCOUNT UNDERRUN bitfields add up to less than 32
+ * ULONG VALUE NEGATIVE attempt convert -ve integers to ulong
+ * ULONG VALUE OVERFLOW converted value does not fit in result
+ * TUNTAP IFNAME LENGTH tun/tap interface name too long
+ * TUNTAP MTU OVERRUN tun/tap mtu limited to 2^16 bytes
+ *