code: func (head *Head) Vars() VarSet {
  	vis := &VarVisitor{vars: VarSet{}}
  	// TODO: improve test coverage for this.
  	if head.Args != nil {
  		Walk(vis, head.Args)
  	}
  	if head.Key != nil {
  		Walk(vis, head.Key)
  	}
  	if head.Value != nil {
  		Walk(vis, head.Value)
  	}
  	return vis.vars
  }
docs: // Vars returns a set of vars found in the head.