GRASS source upgrade and distribution rights?

I read in the grass.intro file that the source is in the public
domain. Is there is any more info on source upgrade and/or
distribution rights?
   
For instance, could GRASS 4+ be freely redistributable via an
actual General Public License?

T. Turner, tturner@gnu.ai.mit.edu
FSF/Project GNU Volunteer
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Cambridge, MA 02139
(617) 253-8568