code: func (expr *Expr) Operator() Ref {
  	terms, ok := expr.Terms.([]*Term)
  	if !ok || len(terms) == 0 {
  		return nil
  	}
  	return terms[0].Value.(Ref)
  }
docs: // Operator returns the name of the function or built-in this expression refers
  // to. If this expression is not a function call, returns nil.