But there's no facility to select elements that are unique to each set, i.e. select elements from each set which aren't present in the other sets

There is. '100' selects elements that are unique to the first set. '001|010|100' selects what you want.

then a lispish sort of query specification might do the job, as used in LDAP queries - (|(&(1=0)(!(2=-1)))(3=1)(4=1)) - perhaps translated into "and", "or", "in", "not in" and such to make it more perlish. Or "and", "or", "not", "nor", "xor" etc - basically, you need boolean logic and precedence to process things, and a descriptive input specification which supports that.

This is precisely what I didn't want to do. Lot of work, lot of edge cases, lot of tests and documentation to write.