Summary:
  - mdo expressions are enabled by RecursiveDo pragma

  - mdo expressions perform full segmentation

  - 'rec' groups inside 'do' are changed so they do *not*
    perform any segmentation.

  - Both 'mdo' and 'rec' are enabled by 'RecursiveDo'
    'DoRec' is deprecated in favour of 'RecursiveDo'
    (The 'rec' keyword is also enabled by 'Arrows', as now.)

Thanks to Levent for doing all the work