Problem
-------
          
        In this assignment  we will learn to access the  details of a
        call graph (such  as indentifying callers or callees  of a given
        function).  For this  purpose, you  have  to write  a plugin  to
        traverse the call graphs and

        a. print the callers of every function,
        b. print the callees of the `main' function (but not other 
           functions).


Procedure
---------

        1. Iterate over call graph (structure variable `cgraph_node',
           details are in file  $SOURCE_D/gcc/cgraph.h). We have already
           seen how to iterate over cgraph in assignment 1.

        2. In  order to find  the list  of callers, iterate  over the
           structure represented by the following field

           `struct  cgraph_edge *callers'

        3. In  order to find  the list  of callees, iterate  over the
           structure represented by the following field
 
            `struct  cgraph_edge *callees'

        4. In  order to  find  out  whether a  node  in  the call  graph
           represents the  `main' function,  use the  macro MAIN_NAME_P.
           Details   of  this   macro   can  be   found   in  the   file
           $SOURCE_D/gcc/tree.h).

        As a  general tip, you should  use cscope and search  for the
        uses  of the  data structures,  APIs  and macros  in the  source
        files. Examining some usages gives valuable information.