code: func interpolationFuncFormat() ast.Function {
  	return ast.Function{
  		ArgTypes:     []ast.Type{ast.TypeString},
  		Variadic:     true,
  		VariadicType: ast.TypeAny,
  		ReturnType:   ast.TypeString,
  		Callback: func(args []interface{}) (interface{}, error) {
  			format := args[0].(string)
  			return fmt.Sprintf(format, args[1:]...), nil
  		},
  	}
  }
docs: // interpolationFuncFormat implements the "format" function that does
  // string formatting.