Procedure ShellSort;

    This completes a shell sort  on  the  list.   This uses the procedure
    variable UserSort which must be defined before calling ShellSort.

    UserSort is of the type

      SorterType  = Function(FirstItem,SecondItem:ListData):Boolean;

    Thus you can  write  your  own  comparison  function  which  compares
    your data  and  returns  True  if  the  items  are  to  be swapped or
    False if the items are not to be swapped.

    FirstItem always  lies  before  SecondItem  when  your  procedure  is
    called.