How to remove dups from 2 lists?

I'm looking for a function that will eliminate all matching
items, like canceling terms in a fraction. If the first list
has 3 "a"s, and the second list has 5 "a"s, then afterwards
the first list should have 0 "a"s and the second list should
have 2 "a"s.

class Array
  def remove_instances( *values )
    result = dup
    values.flatten.each{ |value|
      if i = result.index( value )
        result.delete_at( i )

a1 = %w| a a a b b c |
a2 = %w| a a a a a b |

p a1.remove_instances( a2 )
#=> ["b", "c"]
p a2.remove_instances( a1 )
#=> ["a", "a"]


From: Mike Steiner []