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This is a base class for elements that process and/or transform data by relying upon another program/process to do the actual processing. This process is presented the data coming into the element on its stdin, and the element will send out what it receives from the process' stdout. It provides for most of the details such as setting up sink and src pads, state changes, etc. An inheriting element need only concern about providing for the proper pad templates and (optionally but recommended) implementing a custom set_caps. This call-back is called during (initial and only) caps negotiation, and only upon return the child process is actually started. This typically gives the inheritor a chance to provide the right arguments to the sub-program for the circumstances and data at hand.