   EXPORT_SET(bits,on,off[,separator[,number_of_bits]])
Returns a string in which for every bit set in the value bits, you
get an on string and for every reset bit you get an off
string.  Bits in bits are examined from right to left (from low-order
to high-order bits). Strings are added to the result from left to right,
separated by the separator string (default ','). The number of
bits examined is given by number_of_bits (default 64).
