https://groups.google.com/forum/#!topic/usd-interest/qg898UklVyo
https://graphics.pixar.com/usd/docs/api/sdf_page_front.html
https://graphics.pixar.com/usd/docs/api/sdf_2notice_8h.html#class_sdf_notice
https://github.com/PixarAnimationStudios/USD/blob/master/pxr/base/lib/tf/testenv/notice.cpp