Thanks to Dave Abrahams for pointing out some deficiencies in the transformer filter: - `apply()` has been renamed to `map()`. - `filter()` applies a normal stream filter to the selection. - To reduce confusion with normal stream filter pipelines the `__or__` operator, which previously applied a custom transform, has been renamed to `apply()`. Also added a `substitute()` transformation that applies regex replacement to `TEXT` events.

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