```# MAIN QUESTION

# Is there a nice and fast way to do set operations on arrays of
# non-trivial objects.

# BACKGROUND

# Set operations are beatiful:

a = [1,2,3,4,5]
b = [2,4,6,8,10]

(a & b)
# => [2,4]

# When comparing lists, which is more or less what administration of
# pension schemes is all about, this is very, very nice.

# I very often use stuff like
(a - (a&b))

# => [1,3,5]

# The array library is blazingly fast, I'm really happy with it.

# The challenge arises when my lists don't comprise of fixnums, but instead of
# more specified objects. To simplify:

class Person
def initialize(name, ssid)
@name = name
@ssid = ssid
end

end

list = []
list << Person.new('Peter Zapffe', 1)
list << Person.new('Peter Pan', 2)
list << Person.new('Saint Peter', 3)

# Let's say I want to UNION that to (b = [2,4,6,8,10]) from above, using ssid
# as key.It should in that case return the 'Peter Pan'-person, since he is the
# only one with an ssid included i the list.

(list & b)
# => []

# I can do stuff like:

list_ssid = list.collect{|p| p.ssid}
# => [1,2,3]

union = list_ssid & b
# => [2]

list.select{|p| union.include? p.ssid}
# => [#<Person:0x2b8b300 @name="Peter Pan", @ssid=2>]

# This works, and correctness is always nice, but it doesn't scale very nice,
# basicly because  union.include? scans the union-list from scratch every time.

# I have implemented this before, by sorting both lists and stepping through
# them one at a time. That's still correct and much faster, but I have
a feeling
# it's possible to do it in a much more elegant way, using some sort of
# set-operations.

# I would appreciate any hints and or pointers.