Sample method with requirements on the argument. You can also define post conditions via ensure: { ... }

contracts.fy

# usage:
class Fixnum {
  def / other {
    require: @{
      other class is: Fixnum
      other is > 0
    } body: {
      # actual method body goes here..
    }
  }
}

f = 10 / 2
g = 10 / 0 # requirement error