As long as the expression passed to the function is a valid R expression (e.g., no unbalanced parenthesis),
it can contain arbitrary function names not defined in R. Variables in the expression passed
to `FunctionXform` are always assumed to be fields, and not substituted. That is, even if `x` has a value in the
R environment, the resulting expression will still use `x`.
An expression such as `foo(x)` is treated as a function `foo` with argument `x`. Consequently, passing in an
R vector `c(1,2,3)` to `functionToPMML()` will produce PMML where `c` is a function and `1,2,3` are the arguments.
An expression starting with '-' or '+' (for example, "-3" or "-(a+b)") will be treated as if there is a 0
before the initial '-' or '+' sign. This makes it possible to represent expressions that start with a sign,
since PMML's '-' and '+' functions require two arguments. The resulting PMML node will have a constant 0 as a child.