public Builder<G, C> executor(final Executor executor) {
			_executor = requireNonNull(executor);
			return this;
		}
The executor used by the engine.

@apiNote
If no dedicated {@link Evaluator} is defined, this is also the
executor, used for evaluating the fitness functions.

@param executor the executor used by the engine
@return {@code this} builder, for command chaining
jenetics/src/main/java/io/jenetics/engine/Engine.java