This is necessary to create custom object as mapping instead of Hash.
For example, if you prefer `hashobj.key` instead of `hashobj['key']`:

    ## allows `h.foo` instead of `h['foo']`
    class MagicHash < Hash
      def method_missing(method, *args)
        return super unless args.empty?
        return self[method.to_s]
      end
    end

    ## override to generate MagicHash instead of Hash
    class MagicVisitor < Psych::Visitors::ToRuby
      def empty_mapping(o)
        MagicHash.new
      end
    end

    ## example to access `ydoc.foo` instead of `ydoc['foo']`
    input = <<'END'
    tables:
      - name: admin_users
        columns:
          - name:  id
            type:  int
            pkey:  true
    END
    tree = Psych.parse(input)
    visitor = MagicVisitor.create
    ydoc = visitor.accept(tree)
    p ydoc.tables[0].columns[0].name   #=> "name"
    p ydoc.tables[0].columns[0].type   #=> "int"

This is necessary to generate custom object as mapping instead of Hash
according to context. For example:

    TableObj  = Struct.new('TableObj', 'name', 'columns')
    ColumnObj = Struct.new('ColumnObj', 'name', 'type', 'pkey')

    class CustomVisitor < Psych::Visitors::ToRuby
      def initialize(*args)
        super
        @key_path = []   # ex: [] -> ['tables'] -> ['tables', 'columns']
      end
      def accept_key(k)    # push keys
        key = super k
        @key_path << key
        return key
      end
      def accept_value(v)  # pop keys
        value = super v
        @key_path.pop()
        return value
      end
      def empty_mapping(o)  # generate custom object instead of Hash
        case @key_path.last
        when 'tables'   ; return TableObj.new
        when 'columns'  ; return ColumnObj.new
        else            ; return super o
        end
      end
    end

    ## example to generate custom object according to context
    input = <<'END'
    tables:
      - name: admin_users
        columns:
          - name:  id
            type:  int
            pkey:  true
    END
    tree = Psych.parse(input)
    visitor = CustomVisitor.create
    ydoc = visitor.accept(tree)
    p ydoc['tables'][0].class                #=> Struct::TableObj
    p ydoc['tables'][0]['columns'][0].class  #=> Struct::ColumnObj

This new class allows user to generate custom object instead of Hash.
See document of CustomClassVisitor class for details.