Scala preconditions

## assert
val rnd = Math.random()
val n = Math.abs(rnd)
assert(n > 0)

## assume
val speedFromSensor: Double = ...
assume(speedFromSensor >= 0)

## require_ensuring
def doublePositive(n: Int): Int = {
  require(n > 0)
  n * 2 // logic
} ensuring(n => n >= 0 && n % 2 == 0)
	val speedFromSensor: Double = ...
	assume(speedFromSensor >= 0)
	def doublePositive(n: Int): Int = {
	require(n > 0)
	n * 2 // logic
	} ensuring(n => n >= 0 && n % 2 == 0)