doc-src/Tutorial/Misc/Sep2.thy

Added an exception handler and error msg.

Sep2 = List +
consts sep :: "'a * 'a list => 'a list"
recdef sep "measure (%(a,xs). length xs)"
    "sep(a, x#y#zs) = x # a # sep(a,y#zs)"
    "sep(a, xs)     = xs"
end