class Foo
　def initialize
　　@hash = {}
　end
　def []= (key, value)
　　@hash[key] = value if yield(key)
　end
　def [] (key)
　　@hash[key]
　end
end

foo = Foo.new
pr = proc{|k| k.is_a?(Symbol) ? true : false}

foo.__send__("[]=", "a", "b", &pr)
foo.__send__("[]=", :a, :b, &pr)

p foo["a"] #=> nil
p foo[:a] #=> :b