assertions.rb

module Assertions
  def demand(&block)
    block.call or raise ConditionFailed
  end

  def promise(&block)
    tap { |o| yield o or raise ConditionFailed }
  end

  def refuse(&block)
    ! block.call or raise ConditionFailed
  end

  class ConditionFailed < Exception
  end
end

gistfile1.rb

    def block(vendor)
      demand { vendor.present? }

      blocked_vendors_x.
       block(vendor).
       promise { blocking?(vendor) }
    end