insane.rb

def MYCLASS(klass, &block) 
  eval "class #{klass}; end"
  eval(klass).instance_eval(&block)
end

def MYDEFINE(meth, &proc)
  define_method(meth, &proc)
end

#### Your new DSL: ####

MYCLASS(:MyClass) {
  MYDEFINE(:foo) {|a, b=1|
    puts "hi"
  }
}

MyClass.new.foo