math: passing function to function

[f97stdi@dd.chalmers.se (Staffan Dittmer)]

I'm trying to implement a root finder using the Brent method
but find myself getting stuck. 
I cannot figure  out how to pass a function 
- the function to solve - as a parameter 
to  the solver.

I've tried using access to subprogram types to make something like
function solver(fun: access to function to solve; ...) return long_float;
which usually compiles ok, but exits raising 
constraint error as soon as the code gets more involved than the 
rudimentary examples given in the RM.

I would very much appreciate it if someone could point me to 
some mathematical Ada source code where things like the above 
are bound to be used. 
 
The latest line of thought is that it might be possible to
make a generic construction like

generic
function fun(X:float) return float renames function_to_solve;
function solver(...) return float is
begin 
......

but I have no greater hopes for this being even remotely possible.

Any other ideas?

/ Staffan Dittmer