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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _check_em_conversion(unit, to_unit=None, unit_system=None, registry=None):
 """Check to see if the units contain E&M units This function supports unyt's ability to convert data to and from E&M electromagnetic units. However, this support is limited and only very simple unit expressions can be readily converted. This function tries to see if the unit is an atomic base unit that is present in the em_conversions dict. If it does not contain E&M units, the function returns an empty tuple. If it does contain an atomic E&M unit in the em_conversions dict, it returns a tuple containing the unit to convert to and scale factor. If it contains a more complicated E&M unit and we are trying to convert between CGS & MKS E&M units, it raises an error. """

    em_map = ()
    if unit == to_unit or unit.dimensions not in em_conversion_dims:
        return em_map
    if unit.is_atomic:
        prefix, unit_wo_prefix = _split_prefix(str(unit), unit.registry.lut)
    else:
        prefix, unit_wo_prefix = "", str(unit)
    if (unit_wo_prefix, unit.dimensions) in em_conversions:
        em_info = em_conversions[unit_wo_prefix, unit.dimensions]
        em_unit = Unit(prefix + em_info[1], registry=registry)
        if to_unit is None:
            cmks_in_unit = current_mks in unit.dimensions.atoms()
            cmks_in_unit_system = unit_system.units_map[current_mks]
            cmks_in_unit_system = cmks_in_unit_system is not None
            if cmks_in_unit and cmks_in_unit_system:
                em_map = (unit_system[unit.dimensions], unit, 1.0)
            else:
                em_map = (None, em_unit, em_info[2])
        elif to_unit.dimensions == em_unit.dimensions:
            em_map = (to_unit, em_unit, em_info[2])
    if em_map:
        return em_map
    if unit_system is None:
        from unyt.unit_systems import unit_system_registry

        unit_system = unit_system_registry["mks"]
    for unit_atom in unit.expr.atoms():
        if unit_atom.is_Number:
            continue
        bu = str(unit_atom)
        budims = Unit(bu, registry=registry).dimensions
        try:
            if str(unit_system[budims]) == bu:
                continue
        except MissingMKSCurrent:
            raise MKSCGSConversionError(unit)
    return em_map