This took a few days of overwork and obsessive overexcitement to write. A #Fortran port of my #RosettaCode for #ContinuedFractions, including a primitive #MultiplePrecision module:

Continued fraction/Arithmetic/G(matrix ng, continued fraction n1, continued fraction n2) - Rosetta Code https://rosettacode.org/wiki/Continued_fraction/Arithmetic/G(matrix_ng,_continued_fraction_n1,_continued_fraction_n2)#Fortran

Part of the problem, as I have mentioned before, is that gfortran is not especially helpful in tracking down bugs. But I see gdb has gotten better with Fortran.