def exec_in_proxy_context(dsl, fallback_dsl, proxy_type, *args, &block)
      block_context = fallback_dsl
      proxy_context = proxy_type.new(dsl, block_context)
      begin
        block_context.instance_variables.each do |ivar|
          value_from_block = block_context.instance_variable_get(ivar)
          proxy_context.instance_variable_set(ivar, value_from_block)
        end
        proxy_context.instance_exec(*args, &block)
      ensure
        block_context.instance_variables.each do |ivar|
          value_from_dsl_proxy = proxy_context.instance_variable_get(ivar)
          block_context.instance_variable_set(ivar, value_from_dsl_proxy)
        end
      end
    end
def exec_in_proxy_context(dsl, fallback_dsl, proxy_type, *args, &block)
      block_context = fallback_dsl
      proxy_context = proxy_type.new(dsl, block_context)
      begin
        block_context.instance_variables.each do |ivar|
          value_from_block = block_context.instance_variable_get(ivar)
          proxy_context.instance_variable_set(ivar, value_from_block)
        end
        proxy_context.instance_exec(*args, &block)
      ensure
        block_context.instance_variables.each do |ivar|
          value_from_dsl_proxy = proxy_context.instance_variable_get(ivar)
          block_context.instance_variable_set(ivar, value_from_dsl_proxy)
        end
      end
    end
Execute a block in the context of an object whose methods represent the
 commands in a DSL, using a specific proxy class.

 @param dsl          [Object] context object whose methods make up the
                              (initial) DSL
 @param fallback_dsl [Object] context object that the DSL should fall back
                              to if the primary context fails to resolve
 @param proxy_type   [FallbackContextProxy, ChainingFallbackContextProxy]
                              which class to instantiate as proxy context
 @param args         [Array]  arguments to be passed to the block
 @param block        [Proc]   the block of DSL commands to be executed
 @return             [Object] the return value of the block