The arcs on the tip (around the addendum) and the root (between the
fillets) used to have their bulge in the wrong direction for the internal
gear profile: their center points has been on the outside.
Since this error existed since the very first introcution of the internal
gear I assume this was done by accident: Instead of only the fillets, all
arcs have been inverted when the external profile was copied.
This commit now inverts the tip and root arcs again, to be concentric
with the pitch circle -- same as for the external profile.
ecd660ed7e