Tag
Tag is a smuggler operator. It only allows to name expectedValue,
which can be an operator or a value. The data is then compared
against expectedValue as if Tag was never called. It is only useful
as JSON operator parameter, to name placeholders. See JSON
operator for more details.
TypeBehind method is delegated to expectedValue one if
expectedValue is a TestDeep operator, otherwise it returns the
type of expectedValue (or nil if it is originally untyped nil).
See also Tag godoc.