Add -public flag that will publicly expose a rule for parsing.

[otac0n]

Example:

program = _ expression<0,,_ ';' _> _ EOF

Expression -public = _ e:expression _ { e }

expression = '' [a-z]+

Here, program would still be the start rule and a method called ParseExpression would be exposed.

A warning will be emitted if the -public flag is added to a rule whose name starts with a lower case letter.

[otac0n]

This is a bit half-baked.

For example, what exactly should the method signatures be?
Maybe some combination of these:

IParseResult<T> ParseExpression(ref Cursor cursor);
IParseResult<T> ParseExpression(string subject, string fileName = null);
IParseResult<T> ParseExpression(string subject, string fileName = null, int offset = 0);
T ParseExpression(string subject, string fileName = null);
T ParseExpression(string subject, string fileName = null, int offset = 0);
T ParseExpression(string subject, string fileName, out IList<LexicalElement> lexicalElements);
T ParseExpression(string subject, string fileName, int offset, out IList<LexicalElement> lexicalElements);

Obviously, not all are possible. Do we need to satisfy all of these contracts? What are our actual use cases?

[otac0n]

One known use case: #25 Composite Grammars.

This would require an interface such as IParseResult<T> ParseExpression(ref Cursor cursor) in order to pass control on to the next parser.

[otac0n]

Another possible use case: #32 Higher-order Rules

Higher order rules will need a delegate-passing interface and will benefit most from the same interface as Composite Grammars, IParseResult<T> ParseExpression(ref Cursor cursor).

[david-pfx]

I would use -public to gain access to a rule to use for restarting the parse. The signature need not change. At present I do this via the partial class.

[otac0n]

This is now complete in the develop branch.

I also included an export flag.

The difference is that the public flag implies a contract like the existing Parse method, whereas the export flag implies a signature that is compatible with #parse{} code blocks.