Transcribed Earley's original description for each of the three
primitive steps.
Added field to EarleyComputation to hold the stashed back-pointers.
Revised back-ptr API to add a list of ptrs; fixed bug in API;
incorporated into completor.
Hacked up parse tree reconstruction.

Adapting pointer-semantics to functional style was non-trivial.
Put in tree reconstruction.

Am a little worried about this though:

    > (construct-derivation-trees (ec-back-pointers (earley-compute '((S -> S S "a") (S ->)) 0 "aa")))
    ()

(I think I am not stashing away the final accepted state properly.)
Checkpoint. Should start either composing or other langs soon.
Merge branch 'blog.pnkfx.org' of github.com:pnkfelix/pnkfx-blog into blog.pnkfx.org

Conflicts:
	source/_posts/variety-show-time.org