Index: tests/collections/string-overwrite.cfa
===================================================================
--- tests/collections/string-overwrite.cfa	(revision 3f631d63dc8a3d8d5bd5e664c971d336878cb507)
+++ tests/collections/string-overwrite.cfa	(revision 1abcec9b6adf2ed1d9f833ccc39e3e9f529c3d52)
@@ -73,4 +73,5 @@
 */
 
+const char * OUT_DELIM = "------------------------------------------------------------------------";
 
 void showOneReplacement(string & s, int ms, int ml, int ws, int wl, const char* replaceWith) {
@@ -232,5 +233,5 @@
     };
     for ( i; sizeof(cases)/sizeof(cases[0]) ) {
-        sout | "------------------------------------------------------------------------" | cases[i].label;
+        sout | OUT_DELIM | cases[i].label;
         string replaceIn = alphabetTemplate;
         showOneReplacement( replaceIn, cases[i].ms, cases[i].ml, cases[i].ws, cases[i].wl, cases[i].replaceWith );
@@ -246,4 +247,20 @@
 
 
+    runReplaceCases();
+
+
+    // in `s(i, k) = sr`:
+    // - whlln, whole-string length = len(s)
+    // - sctln, selection length = k
+    // - rplln, replacement length = len(sr)
+
+    // Extra cases via syntax that appears more direct.
+    // Indirect, as drives the table:
+    //    string replace = src(strt, len)`share;
+    //    replace = "xyz";
+    // Direct, following:
+    //    src(strt, len) = "xyz";
+    sout | OUT_DELIM | "syn-direct";
+
     //          0         1         2
     //          01234567890123456789012345
@@ -251,12 +268,88 @@
 
     s(5,5) = "qqqqq";  // start=5, end=10, len=5
-
     sout | s;
 
-
     s(5,0) = "-----";  // start=5, end=5, len=0
-
     sout | s;
 
-    runReplaceCases();
+
+    // Extra cases for at-/out-of-bound access
+    // Every selection starts and ends at a position between characters, inclusive of before-first and after-last
+    // Intended behaviour is
+    // - negative start means "back from whlln;" i.e., -1 is before-last
+    // - negative sctln menas "back from start;" i.e., -1 from middle selects 1 character before start
+    // - squish inward at subject string's first/last character
+    //   - start > whlln means same as start == whlln: after last character, with sctln >= 0 selecting zero characters
+    //   - start < -whlln means same as start == -whlln: before first character, with sctln <= 0 selecting zero characters
+    //   - (let xstart be the position in [0,whlln] equivalent to start, by the above, in the following...)
+    //   - xstart + sctln > whlln means same as sctln = whlln - xstart  (significant when sctln is positive)
+    //   - xstart + sctln < 0 means same as sctln = -xstart  (significant when sctln is negative)
+    // - start = 0 means
+    //   - before first, when sctln is zero or positive
+    //   - after last, when sctln is negative
+    // - start "outside the string" cases are:
+    //   - start = 0 can select forward from first or backward from last (as just stated); note the asymmetric "sctln = 0" handling
+    //   - (so, the following cases are available to specify front-v-back explicitly...)
+    //   - start >= whlln can select only backward from last, i.e. selects final epsilon when sctln >= 0
+    //   - start <= -whln can select only forward from first, i.e. selects initial epsilon when sctln <= 0
+    // most selections have two equivalent representations
+    sout | OUT_DELIM | "boundary";
+/*
+The test
+
+base = 01234
+
+
+(Start|Step) is (just over|right on|just under) the (min|mid|max) value
+|
+whlln (>|=|<) rplln
+|
+0 (=|<) (whlln|rplln)
+
+Every case needs concrete & independent
+whlln
+rpln
+start
+step, aka sctln
+
+Constraint space is
+sel[Start]
+*
+sel[Step]
+*
+lens1
+*
+lens2
+,
+excluding when the lens pair contradicts
+=
+972 concrete test cases (all combinations)
+
+Toward all pairs
+Each case achieves up to 6 further pairings.
+Best-case, achieved with 81 cases.  (162 permutations / 2 orderings of a pair)
+
+
+
+Start
+just over the min value
+just over the mid value
+just over the max value
+just over the min value
+just over the mid value
+just over the max value
+just over the min value
+just over the mid value
+just over the max value
+
+
+*/
+
+
+    // Extra cases for modifying substring of substring.
+    sout | OUT_DELIM | "compound";
+
+    s = "abcdefghijklmnopqrstuvwxyz";
+    string sx = s(5,5)`share;
+
 }