[Nauty] gentreeg of n=2 vertices

[Kevin Ryde]

In the debian i386 packaged nauty 2.6r7, running "nauty-gentreeg 2" prints

    >A nauty-gentreeg Z=1:1 D=1 n=2
    >Z 2 trees generated in 0.00 sec
    :An
    :An

where I thought preferably it shouldn't duplicate (for say consistency
of counts of trees, etc).

Nosing around the gentreeg.c code I noticed too GENTREEG_MAIN is used
for renaming main() for use in a program, but at the end there's a
#ifdef checking GENG_MAIN to do return vs exit.  Perhaps the latter
would be GENTREEG_MAIN too?