public Builder<G, C> offspringFraction(final double fraction) {
			_offspringFraction = probability(fraction);
			return this;
		}
The offspring fraction.

@param fraction the offspring fraction
@return {@code this} builder, for command chaining
@throws IllegalArgumentException if the fraction is not within the
        range [0, 1].
jenetics/src/main/java/io/jenetics/engine/EvolutionParams.java