p Module.constants.length           # => 88
$snapshot1 = Module.constants
class A
  NAME=:abc

  $snapshot2 = Module.constants
  p $snapshot2.length               # => 90
  p $snapshot2 - $snapshot1         # => ["A", "NAME"]

end
p Module.constants.length           # => 89
p Module.constants - $snapshot1     # => ["A"]
p A.constants                       # => ["NAME"]

p A.class               # => Class
p A.class.ancestors       # => [Class, Module, Object, Kernel]

# No argument: same class method as in 1.8:
Module.constants         # ==> All constants
# One argument: uses the instance method:
Module.constants(true)   # ==> Constants of Module (and included modules)
Module.constants(false)  # ==> Constants of Module (only).

class << Module
  define_method :constants_of_module, Module.instance_method(:constants)
end

# Now use this new class method:
class Module
   COOL = 42
end
Module.constants.include?("COOL")  # ==> false, as you mention
Module.constants_of_module         # ==> ["COOL"], the result you want