subclasses. We've restored that support!


## Version 3.13.0

_2019-02-04_

 *  **This release bumps our minimum requirements to Java 8+ or Android 5+.** Cutting off old
    devices is a serious change and we don't do it lightly! [This post][require_android_5] explains
    why we're doing this and how to upgrade.

const (
	letterIdxBits = 6                    // 6 bits to represent a letter index
	letterIdxMask = 1<<letterIdxBits - 1 // All 1-bits, as many as letterIdxBits
	letterIdxMax  = 63 / letterIdxBits   // # of letter indices fitting in 63 bits
)

// Random number state.
// We generate random temporary file names so that there's a good
// chance the file doesn't exist yet.
var (
	randN  uint32
	randmu sync.Mutex
)