2.4.1 :005 > x = {a: {foo: 'foo'}}.freeze
 => {:a=>{:foo=>"foo"}}
2.4.1 :006 > x.delete(:a)
RuntimeError: can't modify frozen Hash
        from (irb):6:in `delete'
        from (irb):6
        from /home/greg/.rvm/rubies/ruby-2.4.1/bin/irb:11:in `<main>'
2.4.1 :007 > x[:a].delete(:foo)
 => "foo"
2.4.1 :008 > x
 => {:a=>{}}