Serf (n.) was what remained to peasants ruined by Charlemagne's wars: to exchange the position of debtor for that of serf.
A Serf (n.) could grow wealthy precisely because custom froze his duty at two days a week while the productivity of his remaining days kept growing.
Serf (n.) hides the domination of the conditions of production over the producers behind the visible domination of lord over serf — so economic mystification has no room there.
The Serf (n.) who fled treated his servitude as accidental to his personality, freed himself not as a class but alone, and founded only a new estate.
A Serf (n.) who could pay his lord in kind could hoard movable goods — so the runaway serfs were already half burghers.