# Patch method will always defer to any pre-defined method of same name,
# only use Patch implementation if method not already defined.
# An error could still occur if the pre-defined method expects a different
# number of parameters.
# By defining in a module rather than direct patching of the class, the
# patch method won't even be used if method already defined directly as an
# instance method on the class (i.e. not from some other mix-in module).

module Patch
  def a_method(x, y, z)
    return super(x, y, z) if defined? super
    x * y * z
  end
end

module Mixin
  def a_method(x, y, z)
    return x + y + z
  end
end

class A
  def a_method(x, y, z)
    return x + y + z
  end
end

class B
  include Mixin
end

class C
end

A.send :include, Patch
B.send :include, Patch
C.send :include, Patch

puts A.new.a_method(2, 3, 4) # 2 + 3 + 4 = 9 -- use method defined in class
puts B.new.a_method(2, 3, 4) # 2 + 3 + 4 = 9 -- delegate to method from Mixin
puts C.new.a_method(2, 3, 4) # 2 * 3 * 4 = 24 -- use method from Patch