Using Grep {expStr, reverse} with more than one expStr parameter

Hi, this is resolved. Just leaving it here in case someone finds the query useful. 

Query:

I have a chemical formula wave. Let's say the cells have CxHyOr, CxHyNzO, CxHyN, CxHySc, CxHyNzSc, CxHyOrSc, CxHy as values where CHNOS are chemical elements and x,y,z,c,r are numerals.  

I need to Grep to produce a W_Index that: 

-excludes formulas containing both O and S. 

-excludes formulas containing both N and O. ..etc. 

I can do this operation smoothly using the reverse operator if only one element needs to be excluded: Grep/INDX/Q/E={"N",1} formulawave (for example to exclude all formulas with N in them)

However, I cannot figure out from anywhere how to do this if my exclusion parameter includes more than one element. For example, if I want W_Index for only the formulas that contain CHS elements but no N or O. If I were to do {"S",0}, the output will include all of CHS, CHNS, CHOS formulas but I just need CHS. 

There can be messy ways of doing this but can Grep work with more than one exclusion parameter? Or any other inbuilt function in IGOR? 

Sincerely,