original:       3a 1b 3c 2d 1e 3f 2g 
insertion sort: 1b 1e 2d 2g 3a 3c 3f 
merge sort:     1b 1e 2d 2g 3a 3c 3f 
heap sort:      1e 1b 2g 2d 3f 3c 3a 
quicksort:      1b 1e 2d 2g 3c 3f 3a 

Read the letters within each group of equal numbers. If they are in
alphabetical order the sort was stable on this input.
