One embodiment relates to a computer-implemented method to estimate a cardinality of a database query. The casing is configured to accommodate developer therein, the casing extending in a first direction and being formed with an opening. When fully assembled, the stacked marker becomes a multi-unit final assembly that affords one the use of a plurality of different types of markers that are snapped together during non-use and taken apart to select individual markers during use. The POF is installed in the module such that a light receiving surface of a light receiving element is at a distance, d, from an apex of the spherical end surface. The invention further relates to a membrane holder obtained by applying this method, to cassettes for use in the method and in the membrane holder, and to a tube filter in which such a membrane holder is applied.