@default = [b c d e f g h i j

In reply to Random feature:

@default = [b c d e f g h i j k l m n o p q r s t u v w x y z space];
@alt1 = [a c d e f g h i j k l m n o p q r s t u v w x y z space];
@alt2 = [a b d e f g h i j k l m n o p q r s t u v w x y z space];

feature calt {
lookup rotate {
sub @default @default' by @alt1;
sub @alt1 @default' by @alt2;
sub @alt2 @alt2' by @default;
} rotate;
lookup rotate;
} calt;