Pierre Weis wrote:

> the body of f. This operation is trivial if you use a conventional
> beta reducer, but it is surprisingly difficult if you use De Bruijn
> indices.

	Just out of curiousity, what do you mean
by a 'difficult' algorithm?

	To explain my question in slightly more depth: given
some fixed problems with known algorithms, all these algorithms,
in the first instance, have equal 'difficulty', namely,  'trivial':
if the algorithm is known, it can be implemented. (In general,
coding a known algorithm is so easy compared with other programming
tasks that I would classify coding by how laborious it is: the only
'difficulty' involved is staying awake long enough to finish the job :-)

	It is sometimes difficult to _find_ an algorithm for a problem,
and one may say that some algorithms are 'inflexible' in the sense
that small variations in the problem make finding a solution
by considering the 'original' algorithm difficult.

	It may also be hard to tranform a correct algorithm into
a more efficient version.

	Also, it is clear that some algorithms are difficult to
understand. And, some algorithms, coded incorrectly, may be difficult
to debug.
 
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