closure color mix(closure color a, closure color b, float t) {
    return (1.0 - t) * a + t * b;
}

float inbounds(float value, float lower, float upper) {
    return (lower < value) && (upper > value);
}

float threshold(float value, float lower, float upper) {
    return value * inbounds(value, lower, upper);
}