> what kind of set should I use to plot (x -> y) -> z kind of function?
((x -> y) -> z) is not a legal MathType. Intuitively it would mean
a function whose range is z, and whose domain is the set of functions
from x to y. This domain has a very large cardinality. If we let R
be the set of real numbers, then legal VisAD function domains are
R, RxR, RxRxR, and R^n in general. But a domain (x -> y) would be
R^R. Any finite sampling of this domain that would fit in a computer
memory would be very sparse.
Interesting question, though.