def parse(loop_name, str)
      loop = @x12_definition[X12::Loop][loop_name]
      #puts "Loops to parse #{@x12_definition[X12::Loop].keys}"
      throw Exception.new("Cannot find a definition for loop #{loop_name}") unless loop
      loop = loop.dup
      loop.parse(str)
      return loop
    end
def parse(loop_name, str)
      loop = @x12_definition[X12::Loop][loop_name]
      #puts "Loops to parse #{@x12_definition[X12::Loop].keys}"
      throw Exception.new("Cannot find a definition for loop #{loop_name}") unless loop
      loop = loop.dup
      loop.parse(str)
      return loop
    end
Creates a parser out of a definition
 initialize
 Parse a loop of a given name out of a string. Throws an exception if the loop name is not defined.