def METHOD_NAME(blk):
    blk.cost = pyo.Param(
        initialize=0.23,
        doc="NaOCl cost",
        units=pyo.units.USD_2018 / pyo.units.kg,
    )
    blk.purity = pyo.Param(
        mutable=True,
        initialize=0.15,
        doc="NaOCl purity",
        units=pyo.units.dimensionless,
    )
    costing = blk.parent_block()
    costing.add_defined_flow("NaOCl", blk.cost / blk.purity)