code: func (c *Criterion) MatchFunc(match MatchFunc) *Query {
  	if c.query.currentField == Key {
  		panic("Match func cannot be used against Keys, as the Key type is unknown at runtime, and there is no value compare against")
  	}
  
  	return c.op(fn, match)
  }
docs: // MatchFunc will test if a field matches the passed in function