nand2tetris HDL: Getting error "Sub bus of an internal node may not be used" - hdl

Question

I am trying to make a 10-bit adder/subtractor. Right now, the logic works as intended. However, I am trying to set all bits to 0 iff there is overflow. To do this, I need to pass the output (tempOut) through a 10-bit Mux, but in doing so, am getting an error.
Here is the chip:
/**
* Adds or Subtracts two 10-bit values.
* Both inputs a and b are in SIGNED 2s complement format
* when sub == 0, the chip performs add i.e. out=a+b
* when sub == 1, the chip performs subtract i.e. out=a-b
* carry reflects the overflow calculated for 10-bit add/subtract in 2s complement
*/
CHIP AddSub10 {
IN a[10], b[10], sub;
OUT out[10],carry;
PARTS:
// If sub == 1, subtraction, else addition
// First RCA4
Not4(in=b[0..3], out=notB03);
Mux4(a=b[0..3], b=notB03, sel=sub, out=MuxOneOut);
RCA4(a=a[0..3], b=MuxOneOut, cin=sub, sum=tempOut[0..3], cout=cout03);
// Second RCA4
Not4(in=b[4..7], out=notB47);
Mux4(a=b[4..7], b=notB47, sel=sub, out=MuxTwoOut);
RCA4(a=a[4..7], b=MuxTwoOut, cin=cout03, sum=tempOut[4..7], cout=cout47);
// Third RCA4
Not4(in[0..1]=b[8..9], out=notB89);
Mux4(a[0..1]=b[8..9], b=notB89, sel=sub, out=MuxThreeOut);
RCA4(a[0..1]=a[8..9], b=MuxThreeOut, cin=cout47, sum[0..1]=tempOut[8..9], sum[0]=tempA, sum[1]=tempB, sum[2]=carry);
// FIXME, intended to solve overflow/underflow
Xor(a=tempA, b=tempB, out=overflow);
Mux10(a=tempOut, b=false, sel=overflow, out=out);
}

Answer 1

Instead of x[a..b]=tempOut[c..d] you need to use the form x[a..b]=tempVariableAtoB (creating a new internal bus) and combine these buses in your Mux10:
Mux10(a[0..3]=temp0to3, a[4..7]=temp4to7, ... );

Answer 2

Without knowing what line the compiler is complaining about, it is difficult to diagnose the problem. However, my best guess is that you can't use an arbitrary internal bus like tempOut because the compiler doesn't know how big it is when it first runs into it.
The compiler knows the size of the IN and OUT elements, and it knows the size of the inputs and outputs of a component. But it can't tell how big tempOut would be without parsing everything, and that's probably outside the scope of the compiler design.
I would suggest you refactor so that each RCA4 has a discrete output bus (ie: sum1, sum2, sum3). You can then use them and their individual bits as needed in the Xor and Mux10.