Context termination is too aggressive

[SquidDev]

Consider the following expression:

( let x = )

This gets converted into the following token stream:

( let x = $end $in )

As a result, we report on an unexpected end of block, rather than an unexpected closing parenthesis. This is because of the termination rules - ) closes the opening (, and so pops all parent contexts.

We need to keep the termination rules in some form - they're easier to manage than explicitly writing every single state transition in the context system. However, I think it's worth reviewing when implicit tokens are inserted, and if they should be inserted at all.

[SquidDev]

As a note to myself, see section 15 of the F# specification, which is where much of the context system's ideas come from.