> Joi.compile(Joi.ref('foo')).describe()
{ type: 'any',
  flags: { allowOnly: true },
  valids:
   [ { [Function]
       isContext: false,
       key: 'foo',
       path: [Object],
       depth: 1,
       root: 'foo',
       isJoi: true,
       toString: [Function] } ] }

> Joi.compile(Joi.string().when('a', { is: 'foo', then: Joi.forbidden() })).describe()
{ type: 'alternatives',
  flags: { presence: 'ignore' },
  alternatives:
   [ { ref: 'ref:a',
       is: [Object],
       then: [Object],
       otherwise: [Object] } ] }