public Builder<G, C> populationSize(final int size) {
			if (size < 1) {
				throw new IllegalArgumentException(format(
					"Population size must be greater than zero, but was %s.",
					size
				));
			}

			_populationSize = size;
			return this;
		}
The number of individuals which form the population. <i>Default
values is set to {@code 50}.</i>

@param size the number of individuals of a population
@return {@code this} builder, for command chaining
@throws IllegalArgumentException if {@code size < 1}
jenetics/src/main/java/io/jenetics/engine/EvolutionParams.java