I agree with mecej4. 

I agree with mecej4.  external derf states that you will supply an implementation of derf and that you don't want to use the Fortran built-in (erf preferred).   Inclusion of erf in the Fortran intrinsic list is a relatively recent change, so you may still be able to find old compilers which prefer the way you have presented it (but you are then likely to need to link specifically against a library which provides derf).