def send_as_package(obj)
  if obj.respond_to? :package
    packaged = obj.package
  else
    if $DEBUG
      $stderr.puts "Not sure how to package a #{obj.class}."
      $stderr.puts 'Trying generic packager.'
    end
    package = Package.new(obj)
  end
  send(package)
end
def multiply_precisely(a, b)
  if a.is_a? Float or b.is_a? Float
    raise ArgumentError, "I can't do precise multiplication with floats."
  end
  a * b
end
multiply_precisely(4, 5)

multiply_precisely(4.0, 5)

ArgumentError: I can't do precise multiplication with floats.
...

ArgumentError: I can't do precise multiplication with floats.
	from (irb):15:in `multiply_precisely'
	from (irb):20

def append_to_self(x)
  unless x.respond_to? :<<
    raise ArgumentError, "This object doesn't support the left-shift operator."
  end
  if x.is_a? Numeric
    raise ArgumentError, 
	"The left-shift operator for this object doesn't do an append."
  end
  x << x
end 
append_to_self('abc')

append_to_self([1, 2, 3])

append_to_self({1 => 2})

ArgumentError: This object doesn't support the left-shift operator.
...

ArgumentError: This object doesn't support the left-shift operator.
	from (irb):23:in `append_to_self'
	from (irb):33

append_to_self(5)

ArgumentError: The left-shift operator for this object doesn't do an append.
...

ArgumentError: The left-shift operator for this object doesn't do an append.
	from (irb):26:in `append_to_self'
	from (irb):34

5 << 5

module ShiftMeansAppend
  def <<(x)
  end
end
class String
  include ShiftMeansAppend
end
class Array
  include ShiftMeansAppend
end
def append_to_self(x)
  unless x.is_a? ShiftMeansAppend
    raise ArgumentError, "I can't trust this object's left-shift operator."
  end
  x << x
end 
append_to_self 4

ArgumentError: I can't trust this object's left-shift operator.
...

ArgumentError: I can't trust this object's left-shift operator.
	from (irb):48:in `append_to_self'
	from (irb):52

append_to_self '4'