1. A conferencing server, comprising: an interface for receiving a plurality of incoming bitstreams from respective conferencing endpoints and supplying at least one outgoing bitstream to the conferencing endpoints, wherein said incoming and outgoing bitstreams comprise media data, including at least one of: audio data, video data, audiovisual data and screen-sharing data; a memory with a plurality of memory spaces; a processor operable to perform operations selected from a predefined collection of primitives, wherein said predefined collection of primitives is suitable to implement a plurality of mixing strategies, and wherein each operation inputs media data from one or more of said memory spaces and/or outputs media data to one or more of said memory spaces; and a pre-processor adapted to receive a mixing strategy specifying a desired content of the at least one outgoing bitstream in relation to generic incoming bitstreams that are unknown as to their number and/or format and/or activity and requiring at least one additive media mixing step, and repeatedly derive a run list of operations selected from said predefined collection of primitives that realizes the received mixing strategy for the received incoming bitstreams, wherein the pre-processor is adapted to derive the run list dynamically while taking into consideration momentary activity in each received incoming bitstream, wherein the processor is configured to execute said run list for thereby supplying the at least one outgoing bitstream on the basis of said plurality of received incoming bitstreams.