Fix for bug #9189 (https://gna.org/bugs/index.php?9189).
This bug was reported by Neil Syme (bmbnrs at leeds dot ac dot uk).
Two of the functions accessed when the second set of extended model-free formulae - using the
parameters {S2f, tf, S2s, ts} rather than the standard {S2, S2f, tf, ts} - do not exist. This were
changed to the correct functions (hopefully)!
