Code Listing 1: Side-by-side comparison of IF statement structures

Use of NOT
IF condA AND NOT ( condB OR condC ) 
THEN 
   proc1; 
ELSIF condA AND (condB OR condC) 
THEN 
   proc2; 
ELSIF NOT condA AND condD 
THEN
   proc3;
END IF; 
Nesting for clarity
IF condA 
THEN 
   IF (condB OR condC) 
   THEN 
      proc2; 
   ELSE 
      proc1;
   END IF; 
ELSIF condD 
THEN 
   proc3
END IF;