The function that does the transformation. Override this function
if you build your own term transformer. It does not return the result
but instead it puts it on the converted stack using setResult().
Instead it can also enqueue some Builders that will in the end put the
result on the converted stack.
You can always call super.convert() if you do not need to convert
the term. It will still convert the sub-terms. If you do not want to
convert the sub terms, call setResult(term) instead.