doc-src/Tutorial/Recdef/sep2

'oops' made proper;
removed 'kill_proof' (superceded by 'kill');

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