cond(x,a,b,c) or cond(x,a,b)
       Domain x:     -8e+307 to 8e+307 and missing; 0 means false, otherwise interpreted as true
       Domain a:     numbers and strings
       Domain b:     numbers if a is a number; strings if a is a string
       Domain c:     numbers if a is a number; strings if a is a string
       Range:        a, b, and c
       Description:  returns a if x is true and nonmissing, b if x is false, and c if x is missing.
                     returns a if c is not specified and x evaluates to missing.