Computes the element-wise sum of all inputs. The structure (type, dimension, complexity) of the output is inferred from the structure of the input data - all inputs must share their primitive numeric type; all inputs must be of identical dimension or scalar; output complexity is the logical OR of all input complexities and the explicit complexity parameter.
Whilst this inference is usually convenient, it means that the output of the block cannot be created until all inputs have been seen. If this leads to Deadlock in your system, you can allow the block to create its output earlier (even, before seeing any inputs) by giving it enough information in its parameters. In this case, naturally, all inputs must comply with these parameters when they are seen. If you specify all three parameters (complex, dims and type) the block can create its output immediately. This allows, also, the use of the block in configurations with zero inputs (in which case, the outputs are all zero).
M Source Code (against 995)
ans = 1 2 3 ans = 4 5 6 ans = 5 7 9