cannot reopen class and modify include to prepend

cannot-reopen-class-and-modify-include-to-prepend.rb

# If i have a class, which includes a module
# Now i cannot reopen the class and prepend the same module 

# ran on ruby 2.2.3p173 (2015-08-18 revision 51636) [x86_64-darwin14]

class Base; end
module M1; end
module M2; end

class C1 < Base; end

puts C1.ancestors.inspect # [C1, Base, Object, Kernel, BasicObject]

class C1
  include M1
  prepend M2
end

puts C1.ancestors.inspect # [M2, C1, M1, Base, Object, Kernel, BasicObject]

class C1
  prepend M1
end

# still still same as before
# M1 did not get prepended
puts C1.ancestors.inspect # [M2, C1, M1, Base, Object, Kernel, BasicObject]