static SymbolIndex of(final RandomGenerator random) {
		requireNonNull(random);
		return (rule, bound) -> random.nextInt(bound);
	}
Create a new symbol-index object from the given random generator. This
can be used for generating random sentences of derivation-trees.

@param random the random generator used for generating the sentences
@return a new symbol-index object from the given random generator
@throws NullPointerException if the given {@code random} generator is
        {@code null}
jenetics.ext/src/main/java/io/jenetics/ext/grammar/SymbolIndex.java