thawing ice followup

Modifying an() [actually just_an()] to treat "<thickness> ice" and
"frozen <hallucinatory liquid>" as special cases which shouldn't be
prefixed with "a" or "an" affected using something like "shaved ice"
or "frozen yogurt" as named fruit.
 |a) shaved ice
 |b) frozen yogurt (weapon in hand)
now have article "a" preceding them:
 |a) a shaved ice
 |b) a frozen yogurt (weapon in hand)
However, the existing cases
 |c) iron bars
 |d) an iron bars (weapon in hand)
still get item 'c' wrong.  'd' is slightly odd but that's because the
fruit name is ambiguous as to whether it's singular or plural.

src/invent.c

@@ -4359,8 +4359,26 @@ look_here(
         }
     }
 
-    if (dfeature && !skip_dfeature)
-        Sprintf(fbuf, "There is %s here.", an(dfeature));
+    if (dfeature && !skip_dfeature) {
+        const char *p;
+        int article = 1; /* 0 => none, 1 => a/an, 2 => the (not used here) */
+
+        /* "molten lava", "iron bars", and plain "ice" are handled as special
+           cases in an() but probably shouldn't be; don't rely on that */
+        if (!strcmp(dfeature, "molten lava")
+            || !strcmp(dfeature, "iron bars")
+            || !strcmp(dfeature, "ice")
+            || !strncmp(dfeature, "frozen ", 7) /* ice while hallucinating */
+            /* thawing ice ("solid ice", "thin ice", &c) */
+            || ((p = strchr(dfeature, ' ')) != 0 && !strcmpi(p, " ice")))
+            article = 0;
+        if (article == 1)
+            dfeature = an(dfeature);
+
+        /* hardcoded "is" worked here because "iron bars" is actually
+           "set of iron bars"; use vtense() instead of relying on that */
+        Sprintf(fbuf, "There %s %s here.", vtense(dfeature, "are"), dfeature);
+    }
 
     if (!otmp || is_lava(u.ux, u.uy)
         || (is_pool(u.ux, u.uy) && !Underwater)) {