The variable id is a cycle as this is more convenient
than a zero-by-one matrix.Function is.id() returns a Boolean with TRUE if the
corresponding element is the identity, and FALSE otherwise. It
dispatches to either is.id.cycle() or is.id.word() as
appropriate.Function is.id.list() tests a cyclist for identityness.
Details
The identity permutation is problematic because it potentially has zero size.