public Builder<T> functions(final Predicate<? super T> functions) {
			_functions = requireNonNull(functions);
			return this;
		}
Set the predicate which defines function tokens.

@param functions the function predicate
@return {@code this} builder, for method chaining
@throws NullPointerException if the {@code functions} is {@code null}
jenetics.ext/src/main/java/io/jenetics/ext/internal/util/FormulaParser.java