Find a prefix character string which is different from all the names currently in use within the SLP problem

The following example reads a problem from file and then finds a unique prefix so that new names can be added without fear of duplications:

p.read ("Matrix", "")
p.setuniqueprefix ()
s = p.attributes.xslp_uniqueprefix
print ("No names start with ", s)

The unique prefix may be more than one character in length, and may change if new names are added to the problem. The value of the unique prefix can be obtained from the string attribute xslp_uniqueprefix.