parse static method
Implementation
static CSSFunction? parse(String? s) {
CSSFunction? cssF = CSSCalc.parse(s);
if (cssF != null) return cssF;
cssF = CSSMax.parse(s);
if (cssF != null) return cssF;
cssF = CSSMin.parse(s);
if (cssF != null) return cssF;
return null;
}