undef singleton method from ruby object

Question

I would like to be able to undef a singleton method on a given object.

class A
end

a = A.new
def a.foo
  puts "bar"
end

# undef a.foo here
a.foo # should crash

Answer 1

class << a
  undef foo
end

Alternatively:

a.singleton_class.send :undef_method, :foo