some comments to describe state of CKTrhs CKTrhsOld at various points

esp, note the split usage
  CKTload() fills the right hand side of the matrix equation into CKTrhs
  then the equation is solved and the result (no longer a right and side)
    is stored in CKTrhs, and finally deployed in CKTrhsOld

src/maths/ni/niaciter.c

@@ -64,6 +64,9 @@ retry:
         }
     } 
     startTime = SPfrontEnd->IFseconds();
+    /* solve(CKTmatrix . x = CKTrhs), then assign CKTrhs <== x
+     * CKTrhsSpare is not used
+     */
     SMPcSolve(ckt->CKTmatrix,ckt->CKTrhs, 
             ckt->CKTirhs, ckt->CKTrhsSpare,
             ckt->CKTirhsSpare);

src/maths/ni/niconv.c

@@ -17,6 +17,9 @@ Author: 1985 Thomas L. Quarles
 extern bool ft_ngdebug;
 
 
+/* here:
+ *   CKTrhs is newest solution, CKTrhsOld the previous solution
+ */
 int
 NIconvTest(CKTcircuit *ckt)
 {

src/maths/ni/niditer.c

@@ -65,6 +65,9 @@ skip:
             return(error); /* can't handle E_BADMATRIX, so let caller */
         }
     } 
+    /* solve(CKTmatrix . x = CKTrhs), then assign CKTrhs <== x
+     * CKTrhsSpare is not used
+     */
     SMPcSolve(ckt->CKTmatrix,ckt->CKTrhs, 
             ckt->CKTirhs, ckt->CKTrhsSpare,
             ckt->CKTirhsSpare);

src/maths/ni/niiter.c

@@ -36,6 +36,10 @@ NIiter(CKTcircuit *ckt, int maxIter)
         maxIter = 100;
 
     if ((ckt->CKTmode & MODETRANOP) && (ckt->CKTmode & MODEUIC)) {
+        /* save current CKTrhs (the last solution) in CKTrhsOld
+         * CKTrhs will be zeroed and then DEVload'ed in CKTload(),
+         *   and later overwritten with the next solution
+         */
         SWAP(double *, ckt->CKTrhs, ckt->CKTrhsOld);
         error = CKTload(ckt);
         if (error)
@@ -52,6 +56,9 @@ NIiter(CKTcircuit *ckt, int maxIter)
 #endif
 
     if (ckt->CKTniState & NIUNINITIALIZED) {
+        /* CKTrhs, CKTrhsOld and CKTrhsSpare
+         *   are zeroed in NIreinit()
+         */
         error = NIreinit(ckt);
         if (error) {
 #ifdef STEPDEBUG
@@ -72,6 +79,8 @@ NIiter(CKTcircuit *ckt, int maxIter)
 #endif
         {
 
+            /* CKTrhs is zeroed, and then DEVload'ed in CKTload()
+             */
             error = CKTload(ckt);
             /* printf("loaded, noncon is %d\n", ckt->CKTnoncon); */
             /* fflush(stdout); */
@@ -159,6 +168,9 @@ NIiter(CKTcircuit *ckt, int maxIter)
                    (size_t) ckt->CKTnumStates * sizeof(double));
 
             startTime = SPfrontEnd->IFseconds();
+            /* solve(CKTmatrix . x = CKTrhs), then assign CKTrhs <== x
+             * CKTrhsSpare is not used
+             */
             SMPsolve(ckt->CKTmatrix, ckt->CKTrhs, ckt->CKTrhsSpare);
             ckt->CKTstat->STATsolveTime +=
                 SPfrontEnd->IFseconds() - startTime;
@@ -189,6 +201,10 @@ NIiter(CKTcircuit *ckt, int maxIter)
                 return(E_ITERLIM);
             }
 
+            /* we have the newest solution CKTrhs,
+             *   and the previous in CKTrhsOld
+             * when calling NIconvTest()
+             */
             if ((ckt->CKTnoncon == 0) && (iterno != 1))
                 ckt->CKTnoncon = NIconvTest(ckt);
             else
@@ -237,6 +253,10 @@ NIiter(CKTcircuit *ckt, int maxIter)
             if (ckt->CKTnoncon == 0) {
                 ckt->CKTstat->STATnumIter += iterno;
                 FREE(OldCKTstate0);
+                /* now copy the newest solution over to CKTrhsOld
+                 *   where it will be expected when returning this function
+                 * CKTrhs won't be used
+                 */
                 SWAP(double *, ckt->CKTrhs, ckt->CKTrhsOld);
                 return(OK);
             }
@@ -266,6 +286,10 @@ NIiter(CKTcircuit *ckt, int maxIter)
         }
 
         /* build up the lvnim1 array from the lvn array */
+        /* save the newest solution in CKRrhsOld
+         * CKTrhs is unused now and ready to be zeroed and DEVload'ed
+         * in the next CKTload() invocation
+         */
         SWAP(double *, ckt->CKTrhs, ckt->CKTrhsOld);
         /* printf("after loading, after solving\n"); */
         /* CKTdump(ckt); */

src/maths/ni/niniter.c

@@ -31,6 +31,9 @@ NInzIter(CKTcircuit *ckt, int posDrive, int negDrive)
 
     ckt->CKTrhs [posDrive] = 1.0;     /* apply unit current excitation */
     ckt->CKTrhs [negDrive] = -1.0;
+    /* solve(CKTmatrix^T . x = CKTrhs), then assign CKTrhs <== x
+     * CKTrhsSpare is not used
+     */
     SMPcaSolve(ckt->CKTmatrix, ckt->CKTrhs, ckt->CKTirhs, ckt->CKTrhsSpare,
 	    ckt->CKTirhsSpare);