Created NonTerminal class for AST nodes that contain other nodes; also
established further notions in AlgebraicPhraseChecker, though it is
incomplete.
Added handler to TerminalSpelling enumeration AlgebraicTerminalSpelling.
Also established boundaries for Clause class, which should probably be
next.
Completed structure for phrase parser. Now it just needs to be debugged.
Current issue (for Algebra Builder, at least) is a terminal sequence of
available operations before a complete program can be built. Solve that,
and you will have a proper Program AST for Algebra.