from skaller:
to make code easier to read, Felix uses Felix names for variables as the C
names.
Unfortunately tut_bind_151 and tut_bind_180 both map C functions
named 'f' to Felix functions. Somehow these are not actually
instantiated in such a way that the name generator registers
the names.
Consequently there's a clash between the C 'f' that is binding
bound, and the 'f' used in List::map by the new 'args' value.
There is workaround, so I renamed library variables _f to make
the problem go away.