config-check

Copperbox revision 3081.

I've changed to sample checkers to use paths defined with reverse function composition (to get a left-to-right reading) and applied with the focus function, rather than use an operator that aliased the focus function.

Using an operator for focus introduced problems with precedence that snagged up the code. Excepting the introduction of a reverse composition operator, checkers look like good old, simple Haskell again (albeit without explicit pattern matching).