Ruby: assert error message produced by a subclass of Set

Question

require 'set'
require 'test/unit'
class Foo < Set
  def to_s
    "to_s"
  end
  alias_method :inspect, :to_s
end

class FooTest < Test::Unit::TestCase
  def test1
    assert_equal(Foo.new, false)
  end
end

Expected output:

test1(FooTest) [test.rb:12]: <to_s> expected but was <false>.

Actual output:

test1(FooTest) [test.rb:12]: <#<Foo: {}>> expected but was <false>.

Edit

Test::Unit uses a strange method called pretty_inspect, which I hear of the first time. Nevertheless, this code will work as expected:

Solution:

require 'set'
require 'test/unit'
class Foo < Set
  def to_s
    "to_s"
  end
  alias_method :pretty_inspect, :to_s
end

class FooTest < Test::Unit::TestCase
  def test1
    assert_equal(Foo.new, false)
  end
end

Answer 1

test/unit appears to not rely on an object's to_s or inspect method. Looking at the source, it may be accessing the object's class inspect method directly, but my attempts to redefine Sets inspect instance method didn't work either. Check out the source of test/unit. ;-)