Begin rewrite.
Typecheck and interpret simple words.
WIP
Typecheck words.
Typecheck and interpret 'empty' and 'if'.
Differentiate scalar and row types.
Typecheck and interpret 'apply'.
Unsafe vector operations.
Fix vector type 'Show' instance.
Kill dead code.
Fix whitespace.
Typecheck and interpret 'fun'.
Typecheck and interpret 'compose'.
Fix style.
Factor repetition.
Reorganize sources and add testing stub.
Test comments.
Test single-character tokens.
Fix whitespace.
Test text.