Subject: query
From: jim stasheff <jds@math.upenn.edu>
Date: Mon, 23 Jul 2007 22:02:07 -0400

In a course on algebraic topolgoy I took from Steenrod, I seem
to recall his treatment of the SECONDARY obstruction to
extension:
Le K^n denote the n-skeleton.
Assume f:K^n --> X extends to K^{n+1}
but that extension does NOT extend to K^{n+2}.
Consider the set of obstructions to extending to K^{n+2}
using all possible extensions to K^{n+1}.

If that doesn't work,
Then retreat to f|K^{n-1} and try again.  Consider the
set of obstructions to extending to K^{n+2}
using all possible extensions to K^n.

Anyone know where this appears in print?

jim