Improving BoundaryToken expansion

[dishmint]

When the boundary is expanded there can be an EndOfLine or EndOfString pattern in the middle of the string expression. Which doesn't make any sense.

[dishmint]

EndOfString might be the thing to avoid, but EndOfLine is probably fine.

[dishmint]

If I keep the boundaries general, as a single symbol for example (BoundaryToken), then I can have rules for simplifying the pattern.

For example if two BoundaryTokens are next to each other, one of them can be dropped.

If BoundaryToken is at the beginning of the pattern, then the expanded pattern can include StartOfString or StartOfLine, etc.