Don't compute a formula when holder already contains value, but splited for different subperiods.
Don't compute a formula when holder already contains value, but splited for different subperiods.

benjello/openfisca-core,adrienpacifico/openfisca-core,openfisca/openfisca-core,sgmap/openfisca-core,benjello/openfisca-core,openfisca/openfisca-core