# The one I basically handed you. Catches duplicates.
$c = which( $a == $b->dummy ) % $a->dim(0);

# Switching up the dims. Basically same as above.
$c = which( $a->dummy == $b ) / $b->dim(0);

# Using summation. Like yours, ignores duplicates.
$c = which( sumover( $a->dummy == $b ) );