0.1. NOTATIO N

IX

denotes th e se t M

K

X

7

. W e will frequentl y writ e expression s o f th e for m

/[[X]CJ]

wher e X i s a se t an d / i s a functio n wit h domai n [X]

CJ.

We use the usual interval notation fo r interval s of ordinals, so, for exam -

ple, i f a (3 then (a , (3) = { 7 (3 | 7 a} an d [a , (3] = { 7 (3 | 7 a}.

As usual , \X\ denote s th e cardinalit y o f X , s o i f s i s a finite sequenc e

then |s | denote s it s length .