| http://www.w3.org/ns/prov#value | - Thus, the pre-filter ring 65 and 80 may for instance be substituted by a perforated or gas permeable ring of relatively rigid plastic, flexible plastic film or cardboard or other suitable, preferably biodegradable material and preferably provided at the proximal edge region thereof with a flexible annular apron resiliently contacting the distal surface of the face plate between the coupling ring 6
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