be_matcher.rb

# What is existence?

class Be
  def initialize
  end

  def matches?(actual)
    defined? actual
  end

  def failure_message
    "expected the expression to exist, but it didn't"
  end

  def negative_failure_message
    "expected the expression to not exist, but it did"
  end
end

# Now you can say things like
# 
# Zen.should be
# 
def be
  Be.new
end

# Note that if Zen does not exist, you won't get a failure message,
# you'll get an undefined constant exception. A pity.