Bala

Copperbox revision 151.

A new implementation of concatenation as a snoc list. This should be used to replace old version in due course. I've left old version in the code for the time being as there aren't any name clashes, though it would be nice to reclaim the (+++) operator.

A useful property of the snoc list version is that the 'nil' type (cNil) is parameterized by its context (the old version had no nil representation). This puts the type checker to work for us, elementCtx and toplevelCtx are differently typed nils:

pitches_001 () = elementCtx `cSnoc` (note _g) `cSnoc` (note _c)

pitches_002 () = toplevelCtx `cSnoc` (note _g) `cSnoc` (note _c)

pitches_001 compiles because notes are at the element context, but pitches_002 fails as notes can't exist at the toplevel context.