code: func (ps *printState) isEmpty(a AST) bool {
  	switch a := a.(type) {
  	case *ArgumentPack:
  		return len(a.Args) == 0
  	case *ExprList:
  		return len(a.Exprs) == 0
  	case *PackExpansion:
  		return a.Pack != nil && ps.isEmpty(a.Base)
  	default:
  		return false
  	}
  }
docs: // isEmpty returns whether printing a will not print anything.