def replace(name, options={}, &block)
      block_definitions.delete(name)
      define(name, options, &block)
    end
def replace(name, options={}, &block)
      block_definitions.delete(name)
      define(name, options, &block)
    end
Define a block, replacing an existing block by the same name if it is already defined.
   <%= blocks.define :some_block_name, :parameter1 => "1", :parameter2 => "2" do |options| %>
     <%= options[:parameter1] %> and <%= options[:parameter2] %>
   <% end %>

   <%= blocks.replace :some_block_name, :parameter3 => "3", :parameter4 => "4" do |options| %>
     <%= options[:parameter3] %> and <%= options[:parameter4] %>
   <% end %>
 Options:
 [+name+]
   The name of the block being defined (either a string or a symbol)
 [+options+]
   The default options for the block definition. Any or all of these options may be overridden by
   whomever calls "blocks.render" on this block.
 [+block+]
   The block that is to be rendered when "blocks.render" is called for this block.