= PropertyHash

The Property hash can be used an object in itself.

     h = PropertyHash.new(:a=>1, :b=>2)
     h[:a]        #=> 1
     h[:a] = 3
     h[:a]        #=> 3

Becuase the properties are fixed, if we try to set a key that is not present,
then we will get an error.

    expect ArgumentError do
      h[:x] = 5
    end

The PropertyHash can also be used as a superclass.

    class MyPropertyHash < PropertyHash
      property :a, :default => 1
      property :b, :default => 2
    end

    h = MyPropertyHash.new
    h[:a]        #=> 1
    h[:a] = 3
    h[:a]        #=> 3

Again, if we try to set key that was not fixed, then we will get an error.

    expect ArgumentError do
      h[:x] = 5
    end

