code: func interpolationFuncIndex() ast.Function {
  	return ast.Function{
  		ArgTypes:   []ast.Type{ast.TypeList, ast.TypeString},
  		ReturnType: ast.TypeInt,
  		Callback: func(args []interface{}) (interface{}, error) {
  			haystack := args[0].([]ast.Variable)
  			needle := args[1].(string)
  			for index, element := range haystack {
  				if needle == element.Value {
  					return index, nil
  				}
  			}
  			return nil, fmt.Errorf("Could not find '%s' in '%s'", needle, haystack)
  		},
  	}
  }
docs: // interpolationFuncIndex implements the "index" function that allows one to
  // find the index of a specific element in a list