[nem-en] Feature request

[vc]

   using System.Console;
   
   variant A
   {
     | X { value : int; }
     | Y
   }
   
   def a = A.X(123) : A;
   
   when (a != null && a is X(value))
     WriteLine(value); // value is accessible in this point
   
   if (a is X(value))
     WriteLine(value); // value is accessible in this point
   else
     WriteLine(":(");