Assignment 2c solution


The basic structure of the auxiliary system in English leads to 
the following schema for sequences ending in a verb:


    (MODAL) (HAVE) (BE) verb

Here parentheses indicate optionality.  This means that all of the
following 8 cases should be accepted:

   write (wrote, writes)
   BE writing
   HAVE written
   CAN write
   CAN BE writing
   HAS BEEN writing
   CAN HAVE WRITTEN
   CAN HAVE BEEN WRITING

where BE indicates multiple forms (is, are, etc.),
CAN covers the various modals (can, could, will),
and HAVE covers the various forms (have, had, etc.)