An error inside a klammer is reported at your document; the image search path

Assembled from development commit 632297b7657a. Since 1b00738: when something
goes wrong while a klammer is being applied, the report begins with the
line of your document that applied it, then lists the klammers being
applied and where inside the klammer set the fault was raised; the
Python binding's line and column are the application's too. The default
image search path is the document's directory, imgsrc beside it and
../imgsrc, and a not-found report shows exactly those.

mac/error.cpp

@@ -15,6 +15,23 @@ bool display_source(const std::string& filename_arg)
     }
 }
 
+// "file, line N, character M" for a location, under the same rule as the
+// headline: the file only when display_source() says so.
+static std::string where(const Locator& loc)
+{
+    std::stringstream ss {};
+    if (display_source(loc.m_filename)) {
+        ss << loc.m_filename;
+    }
+    if (loc.m_line > -1) {
+        ss << (ss.str().empty() ? "line " : ", line ") << loc.m_line;
+    }
+    if (loc.m_chr > -1) {
+        ss << ", character " << loc.m_chr + 1;
+    }
+    return ss.str();
+}
+
 std::string Error::message(const std::string& epilog) const
 {
     // The separator is not cosmetic: the description ends with a sentence, and
@@ -28,16 +45,52 @@ std::string Error::message(const std::string& epilog) const
     }
     std::stringstream ss {};
     ss << command_name << " (" << m_type << " error)";
-    if (display_source(m_loc.m_filename)) {
-        ss << ":\n  " << m_loc.m_filename;
+    // The headline is location(): the outermost application when the error
+    // escaped through klammer applications, which is the user's document.
+    const Locator& head = location();
+    if (display_source(head.m_filename)) {
+        ss << ":\n  " << head.m_filename;
     }
-    if (m_loc.m_line > -1) {
-        ss << ", line " << m_loc.m_line;
+    if (head.m_line > -1) {
+        ss << ", line " << head.m_line;
     }
-    if (m_loc.m_chr > -1) {
-        ss << ", character " << m_loc.m_chr + 1;
+    if (head.m_chr > -1) {
+        ss << ", character " << head.m_chr + 1;
     }
-    ss << ":\n\n" << justify(desc, 80, 0);
+    ss << ":\n\n";
+    // Then the chain, outermost first, each level saying which klammer was
+    // applied and, below the top, where; last, where the error itself was
+    // raised when that is not the innermost application.  Lines begin with
+    // two spaces so justify() keeps them verbatim.  A recursion error
+    // carries hundreds of levels: the outermost, an ellipsis and the two
+    // innermost say what a person needs.
+    if (!m_applications.empty()) {
+        const std::size_t n = m_applications.size();
+        auto level = [&](std::size_t i, bool with_place) {   // i: 0 = innermost
+            ss << "  applying @" << m_applications[i].first;
+            if (with_place) {
+                ss << " at " << where(m_applications[i].second);
+            }
+            ss << "\n";
+        };
+        level(n - 1, false);
+        if (n > 4) {
+            ss << "  ... " << n - 3 << " applications ...\n";
+            level(1, true);
+            level(0, true);
+        } else {
+            for (std::size_t i = n - 1; i-- > 0; ) {
+                level(i, true);
+            }
+        }
+        const Locator& inner = m_applications.front().second;
+        if (m_loc.m_filename != inner.m_filename || m_loc.m_line != inner.m_line
+            || m_loc.m_chr != inner.m_chr) {
+            ss << "  at " << where(m_loc) << "\n";
+        }
+        ss << "\n";
+    }
+    ss << justify(desc, 80, 0);
     return ss.str();
 }