Under Octave, gamma priors fail when variance is too small
This is reproducible on file RBC_Est.mod distributed with the user guide, which has two gamma priors.
The problem is related to a bug in Octave: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=493869
It may also affect beta priors which rely on gaminv() function.
A minimal solution would be to give a sensible error message to the user, so that he knows he has to increase the variance.
A better solution would be to reimplement gaminv.