So if you have odddouble.m with

    function y=odddouble(a,b)
       y=2*x+1
    endfunction

you can do

    >>> h = under(@(x) 1/x, odddouble)
    >>> h(3)

? If so, yeah, I agree you can implement under in matlab (as long as you have the symbolic toolbox as well); in which case it's probably one of very few non-CAS systems where you can define it.