  Revised 1977 seminar handout on Quillen's proof of the 1976 Quillen-Suslin
theorem -- that all finitely generated, projective $k[x_1,\ldots,x_n]$-modules
are free. This particular write-up is based on arguments of Quillen, Paul
Roberts, Vaserstein, and Moshe Roitman -- arguments that are here translated
into the language of idempotent matrices, which seems rather natural.
