Kahn on the dressing DAG
  initial in-degrees: shirt=0 tie=1 jacket=2 belt=2 watch=0 undershorts=0 pants=1 shoes=3 socks=0
  step  1: emit shirt        freed: tie
  step  2: emit tie          freed: -
  step  3: emit watch        freed: -
  step  4: emit undershorts  freed: pants
  step  5: emit pants        freed: belt
  step  6: emit belt         freed: jacket
  step  7: emit jacket       freed: -
  step  8: emit socks        freed: shoes
  step  9: emit shoes        freed: -
  order: shirt tie watch undershorts pants belt jacket socks shoes 
  valid topological order: yes

Kahn on the same graph plus the edge jacket -> undershorts
  vertices emitted: 4 of 9 -- shirt tie watch socks 
  stuck (in-degree never fell to 0): jacket belt undershorts pants shoes
  verdict: cycle detected, no topological order exists
