The support for variable-size objects in AIL is strongly guided by the limitations of C in this matter. Basically, AIL allows what is feasible in C: functions may have variable-size arrays as parameters (both input or output), provided their length is passed separately. In practice this is narrowed to the following rule: for each variable-size array parameter, there must be an integer parameter giving its length. (An exception for null-terminated strings is planned but not yet realized.)
Variable-size arrays in AIL or C correspond to sequences in Python: lists, tuples or strings. These are much easier to use than their C counterparts. Given a sequence object in Python, it is always possible to determine its size: the built-in function len() returns it. It would be annoying to require the caller of an RPC stub with a variable-size parameter to also pass a parameter that explicitly gives its size. Therefore we eliminate all parameters from the Python parameter list whose value is used as the size of a variable-size array. Such parameters are easily found: the array bound expression contains the name of the parameter giving its size. This requires the stub code to work harder (it has to recover the value for size parameters from the corresponding sequence parameter), but at least part of this work would otherwise be needed as well, to check that the given and actual sizes match.
Because of the symmetry in Python between the parameter list and the return value of a function, the same elimination is performed on return values containing variable-size arrays: integers returned solely to tell the client the size of a returned array are not returned explicitly to the caller in Python.