Definition of Constants

   is there a way to define some constants for a session.     

   It would be nice to have that if it is not possible yet.     

   Maybe you can define some standard constants like the speed of light...     

   your example            c  the speed of light,      

                                     e  base of natural log, apprix 2.71828     

                                     i   sqrt(–1) imaginary number     

   an obvious one         π  pi, approx 3.14159     

   _.