class Sass::Tree::Visitors::Perform

def visit_directive(node)
  node.resolved_value = run_interp(node.value)
  old_in_keyframes, @in_keyframes = @in_keyframes, node.normalized_name == "@keyframes"
  with_environment Sass::Environment.new(@environment) do
    node.children = node.children.map {|c| visit(c)}.flatten
    node
  end
ensure
  @in_keyframes = old_in_keyframes
end