MeanGate[TT] {.final.} = ref object of Gate[TT]
SumGate[TT] {.final.} = ref object of Gate[TT]
proc mean[TT](a: Variable[TT]): Variable[TT]
proc mean[TT](a: Variable[TT]; axis: Natural): Variable[TT]
proc sum[TT](a: Variable[TT]): Variable[TT]