code: func interpolationFuncCidrNetmask() ast.Function {
  	return ast.Function{
  		ArgTypes: []ast.Type{
  			ast.TypeString, // CIDR mask
  		},
  		ReturnType: ast.TypeString,
  		Variadic:   false,
  		Callback: func(args []interface{}) (interface{}, error) {
  			_, network, err := net.ParseCIDR(args[0].(string))
  			if err != nil {
  				return nil, fmt.Errorf("invalid CIDR expression: %s", err)
  			}
  
  			return net.IP(network.Mask).String(), nil
  		},
  	}
  }
docs: // interpolationFuncCidrNetmask implements the "cidrnetmask" function
  // that returns the subnet mask in IP address notation.