CellVector lookupCreate(TensorAddress addr) {
            return map.computeIfAbsent(addr, k -> new CellVector());
        }
Maps the content in the given list to the given array, using the given index map.

@return true if the mapping was successful, false if one of the destination positions was
        occupied by a different value
vespajlib/src/main/java/com/yahoo/tensor/functions/Concat.java