default @SideEffectFree boolean isValidBoolean(final boolean liberal) {
        if (liberal) {
            return BOOLEAN_LIBERAL.contains(this.peekString().toUpperCase(Locale.ROOT));
        } else {
            return BOOLEAN_STRICT.contains(this.peekString().toUpperCase(Locale.ROOT));
        }
    }
Returns whether {@link #peekString()} contain a valid {@link Boolean}
within the given range.

@param liberal whether non-strict boolean values like "yes", "no", "on" and "off" should be allowed
@return whether the input until the next whitespace contains a valid {@link Boolean}
cloud-core/src/main/java/org/incendo/cloud/context/CommandInput.java