if (not B) {
   A 
} else {
   if (E and F) {   
      C and D
   } else {         
      undef
   }
}

##</code><code>##

(not B and A)
or
(B and (
   ((E and F) and (C and D))
   or
   (not (E and F) and undef)
))

##</code><code>##

(not B and A) or (B and E and F and C and D)

##</code><code>##

For convertion to single expressions:
if (A) { B } else { C } === (A and B) or (not A and C)

For simplification:
X and undef === undef
X or undef  === X