Halfleads for eighteen+ bells

[peterjscott]

Diagram generator is failing on {n-1/n} halfleads for 18 bells and above.
Only examples currently on database are
Feering Surprise Twenty
-3J-14-12-3J.14-14.3J.14-14.3J.14-14.3J.14-14.3J.14-14.3J.14-14.3J.14-14.3J.14-34.HJ,12
Feering Surprise Eighteen
-3G-14-12-3G.14-14.3G.14-14.3G.14-14.3G.14-14.3G.14-14.3G.14-14.3G.14-34.FG,12
where the diagram does an treble dodge (an X) instead, and then the red line is in a tizzy :-)

This one is OK:
Feering Surprise Sixteen
-3D-14-12-3D.14-14.3D.14-14.3D.14-14.3D.14-14.3D.14-14.3D.14-34.CD,12
and without looking at the code, my guess is that it works OK in this context with C=15, D=16 but thinks F means 18 rather than 17 and G means 19 rather than 18. Similar nastiness with H as 20 rather than 19 and J as 22 rather than 20, maybe - and if the derived places are too big for the number of bells, it sticks with an X (no places made) ??

Regards
PeterScott