The logical functions use 0 to indicate a FALSE condition,
while non-zero values indicate TRUE. Functions return 1 to
indicate TRUE and 0 for FALSE.

OPERATORS: =, <>, <, >, <=, >=

IF(expr1,expr2,expr3): These expressions may be anything,
   including nested IFs. If expr1 evaluates to non-zero,
   then IF returns the result of expr2, but if expr1
   generates 0, IF returns the result of expr3.

AND(expr list): Returns TRUE if all expressions evaluate
   to non-zero, and FALSE if any evaluate to 0.


OR(expr list): Returns TRUE if any of the expressions
   evaluate to non-zero, and FALSE only if all evaluate to
   zero.

AND and OR require at least two arguments, and the upper
   limit is 20.

NOT(expr): Returns TRUE if expr evaluates to zero, and FALSE
   only if expr generates a non-zero value.

ISEMPTY(cell/range list): Returns TRUE if any cells con-
   tained within the list are empty, and FALSE only if all
   possess a value.


ISERR(expr/range list): Returns TRUE if any expressions or
   cells within ranges evaluate to an error condition,
   excluding the NA error status.

ISNA(expr/range list): Similar to ISERR, but this function
   checks specifically for the NA error status.

TRUE(): Returns 1.

FALSE(): Returns 0.





