+ * hbytes hash-prop PROPERTY ALG => property value
+ *
+ * ulong ul2int ULONG => INT can fail if >INT_MAX
+ * ulong int2ul INT => ULONG can fail if <0
+ * ulong mask A B => A & B
+ * ulong add A B => A + B (mod 2^32)
+ * ulong subtract A B => A - B (mod 2^32)
+ * ulong compare A B => 0 -1 (A<B) +1 (A>B)
+ * ulong shift l|r ULONG BITS fails if BITS >32
+ *
+ * ulong ul2bitfields VALUE [SIZE TYPE [TYPE-ARG...] ...] => 0/1
+ * ulong bitfields2ul BASE [SIZE TYPE [TYPE-ARG...] ...] => ULONG
+ * goes from left (MSbit) to right (LSbit) where
+ * SIZE is size in bits
+ * TYPE [TYPE-ARGS...] is as below
+ * zero
+ * ignore
+ * fixed ULONG-VALUE
+ * uint VARNAME/VALUE (VARNAME if ul2bitfields;
+ * ulong VARNAME/VALUE VALUE if bitfields2ul)
+ *
+ * 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