[Haskell] SPJ book - big idea behind supercombinator reduction restriction

Simon Peyton Jones's book The Implementation of Functional Programming Languages, chapter 13 (Supercombinators and Lamba-lifting), page 222, describes a multi-argument beta-reduction strategy whereby several free variables of a multi-argument abstraction may be instantiated simultaneously.

[Haskell] Excessive sharing and GHC

Questions prompted by the Excessive Sharing section, page 405, of SPJ's The Implementation of Functional Programming. This section gives 2 variations of a power list function, semantically equivalent, but with different space behaviours. Paraphrasing, given a caller of length (powerListXXX