Using mpq_t for aggregating currencies with wildly varying ranges

[Donovan Hide]

Thanks Phil and David for the pointers! Much appreciated. I've found that
the simplifying of the rationals hasn't occurred when I've switched over to
using the C interface from the C++ interface. Not sure if a canonicalise
was hidden away somewhere in the C++ wrapper output handling...

David, any chance of seeing your code? I'm just working on the decimal
place shifting in the string output. My C is very rusty :-)