func EditScriptForStrings(source []rune, target []rune, op Options) EditScript {
	return backtrace(len(source), len(target),
		MatrixForStrings(source, target, op), op)
}
func EditScriptForStrings(source []rune, target []rune, op Options) EditScript {
	return backtrace(len(source), len(target),
		MatrixForStrings(source, target, op), op)
}
// EditScriptForStrings returns an optimal edit script to turn source into
// target.