Clarify that ROOT master is not yet production quality.
mention that pythia8 needs to be >= 8180.
libcxx11 should be only turned on by default when using clang and not gcc
Instantiation can create new decls; provide a transaction.
Fix an if condition in TMath::Quantile taking into account numerical errors
(this fixes a wrong result obtained in 32 bit arch , with extended precision)
extend interface of FlexibleInterpVar to change nominal/high/low values