>>>>> "Andrew" == Andrew Welch <andrew.j.welch@xxxxxxxxx> writes:

    Andrew> Out of interest, can someone comment on the cost of
    Andrew> calling these functions?  I imagine element-available() is
    Andrew> very cheap, but I'm wondering about function-available()
    Andrew> with user created extensions...

That is going to depend upon the implementation.
In Gestalt, function-available() is very cheap. User-written functions
are compiled and linked into the executable, and are just like
built-in functions.
-- 
Colin Adams
Preston Lancashire