4.4: improve display of strings in unparse used in programs; fixes so…

…urce-academy#571

xml/chapter4/section4/subsection4.xml

@@ -3183,9 +3183,23 @@ function unparse(exp) {
 }
 	    </JAVASCRIPT>
 	    <JAVASCRIPT_RUN>
+function has_char(x, c) {
+    let found = false;
+    let i = 0;
+    while (char_at(x, i) !== undefined) {
+        found = found || char_at(x, i) === c;
+        i = i + 1;
+    }
+    return found;
+}
+function better_stringify(x) {
+    return is_string(x) &amp;&amp; ! has_char(x, "'")
+           ? "'" + x + "'"
+           : stringify(x);
+}
 function unparse(exp) {
     return is_literal(exp)
-           ? stringify(literal_value(exp))
+           ? better_stringify(literal_value(exp))
            : is_name(exp)
            ? symbol_of_name(exp)
            : is_list_construction(exp)