code: func interpolationFuncFlatten() ast.Function {
  	return ast.Function{
  		ArgTypes:   []ast.Type{ast.TypeList},
  		ReturnType: ast.TypeList,
  		Variadic:   false,
  		Callback: func(args []interface{}) (interface{}, error) {
  			inputList := args[0].([]ast.Variable)
  
  			var outputList []ast.Variable
  			return flattener(outputList, inputList), nil
  		},
  	}
  }
docs: // Flatten to single list