  I apply commonly used regularization schemes to a multi-loop calculation to
examine the properties of the schemes at higher orders. I find complete
consistency between the conventional dimensional regularization scheme and
dimensional reduction, but I find that the four dimensional helicity scheme
produces incorrect results at next-to-next-to-leading order and singular
results at next-to-next-to-next-to-leading order. It is not, therefore, a
unitary regularization scheme.
