[ Utils.Either ] clean: removed old code, pointless __IMPOSSIBLE__, commented.
[ Utils.Either ] new: fromLeft, fromRight.
[ testsuite ] Simplified tests for matching against coinductive constructor.
[ Issue 1339 ] Fixed by checking for coinductive record upon split.
[ Issue 1339 ] interaction test.
[ Coverage ] Export list, removed old code.
[ clean ] Removed old --without-K test from TC.Rules.LHS.Split.

Originally by Nisse, with changes by Andreas, replaced by Jesper's
systematic solution.
[ clean ] Export list for LHS.Split, removed some old code.
[ refactor ] LHS.Split: named fields of SplitProblem.
[ comments ] LHS.Split: comments and spacing.
[ refactor ] LHS.Split: renamed a to dom@(Dom ai a).
[ new ] class Syntax.Internal.SgTel to create singleton telescope.
[ refactor ] LHS.Split: renamed tel to xtel@(Abs x tel).
[ cosmetics ] Nicer expression of Abstract Telescope.
[ refactor ] LHS.Split: direct left-extension of SplitProblem by pattern.
[ Utils.Null ] instances for tuples and Syntax.Internal.Tele.
[ refactor ] Null.empty instead of empty for (Split)Problem.
[ clean ] Removed Monoid instance for Problem(Rest).

This was never a proper monoid, and mappend was only used once.
A saner consSplitProblem gives more comprehensible code.
[ Utils.Except ] Removed Haskell 98 workaround for instance Error String.

We have departed from Haskell 98 long ago...
[ cosmetics ] LHS.Split: moved keepGoing to tryAgain.