'Create a FieldArray of <length> fields of class <elements_class>,
named "<name>[x]".  The **elements_extra_args will be passed to the
constructor of each field when yielded.'

def __init__(self, parent, name, elements_class, length, **elements_extra_args):

    FieldSet.__init__(self, parent, name)
    self.array_elements_class = elements_class
    self.array_length = length
    self.array_elements_extra_args = elements_extra_args
