@Override
	public T apply(final T[] variables) {
		if (_index >= variables.length) {
			throw new IllegalArgumentException(format(
				"No value for variable '%s' given.", this
			));
		}
		return variables[_index];
	}
The projection index of the variable.

@return the projection index of the variable
jenetics.prog/src/main/java/io/jenetics/prog/op/Var.java