st: algebraic with missings

["Roberto De Miguel" <rdemi@indec.mecon.gov.ar>]

Dear statalisters,
  
To solve an equation of the type:  
  
p = a + b + c  
  
where a, b or c can be missing,  
I use the following sentence:  
  
egen p = rsum (a b c)  
  
But now I want to solve:  
  
q = a + b - c  
  
How can I do that?
  
Thank you in advance,  
  
Roberto


*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/