https://github.com/xgqfrms/svg-tutorials
symbol id & use xlink:href
https://www.w3.org/TR/SVG11/struct.html#SymbolElement
https://developer.mozilla.org/en-US/docs/Web/SVG/Element/symbol
https://css-tricks.com/svg-symbol-good-choice-icons/
http://tutorials.jenkov.com/svg/symbol-element.html