doc-src/Tutorial/Recdef/sep

fixed subgoal_tac; fails on non-existent subgoal;

consts sep :: "'a * 'a list => 'a list"
recdef sep "measure (%(a,xs). length xs)"
  "sep(a, [])     = []"
  "sep(a, [x])    = [x]"
  "sep(a, x#y#zs) = x # a # sep(a,y#zs)"