def copy_dimension(source_dataset, target_dataset, name, overwrite=True,
    allow_unlimited=False):
    """
    Copies a dimension to a netCDF file, deleting it if it already exists (unless overwrite is false, in which case
    this raises an exception).
    Copies the size if the dimension is not unlimited.

    :param source_dataset: the source netCDF dataset
    :param target_dataset: the target netCDF to copy into.  Must be in edit / write mode.
    :param name: the name of the dimension
    :param overwrite: if true, overwrite the dimension if found in target.  May cause bad side effects if other variables
    :param allow_unlimited: if true, allow unlimited dimensions to remain unlimited
    depend on that dimension in the target.

    :return new dimension
    """
    if name in target_dataset.dimensions:
        if overwrite:
            del target_dataset.dimensions[name]
        else:
            raise Exception(
                'Target dimension already exists, and overwrite is false')
    source_dimension = source_dataset.dimensions[name]
    if allow_unlimited and source_dimension.isunlimited():
        return target_dataset.createDimension(name, None)
    else:
        return target_dataset.createDimension(name, len(source_dimension))
