Allow multiple-choice questions to have to right answers

gen.pl

@@ -55,7 +55,7 @@ while (<>) {
       $any = 1 if (not (defined $opt->{"correct"})) or ($opt->{"correct"} eq JSON::true);
     }
     die "no correct option at \"$question\" ($ARGV)\n"
-      unless $any;
+      if not($any) and $single_choice;
 
     my %ent =
       (