[idvusers] Adding two different 4d grids

Dear IDV Users,

What is the best way to add two different 4d grids (ie. having different
units) in order to create dimensionless field?

Eg.  Suppose, for whatever reason, you wanted to add convective available
potential energy (CAPE), F1, and rainfall rate, F2.

I have not been able to get either of the following to work (IDV produces

def simpleAdd(F1,F2):
  res = add(F1,F2)
  return res

def simpleAdd(F1,F2):
  res = add(noUnit(F1),noUnit(F2))
  return res

What should I do?