Missing feature:

- Option "-c command" to run `command' and use its PID as filter.

Missing language features:

- if/else
- scratch variable ($name)
- `args', tracepoint arguments support (requires kernel work)
- str()
- @ = hist(x)
- @ = lhist(x, min, max, step)

Improvements:

- implement arithmetic operator precedence
- rewrite stmt_printf() to not use strings internally
- look at collision in map hash