How to get required/optional fields?

[patarapolw]

For example u"Hey {{#who}}{{is}}!{{/who}}"

Should return

{
    "required": [],
    "optional": ["who", "is", "who.is"]
}

[wphakawan]

For example u"Hey {{#who}}{{is}}!{{/who}}"

Should return

{ "required": [], "optional": ["who", "is", "who.is"] } **How can do for next step?