substack LaTeX command support

Sources/SwiftMath/MathRender/MTMathListBuilder.swift

@@ -366,6 +366,13 @@ public struct MTMathListBuilder {
                 // \ means a command
                 assert(!oneCharOnly, "This should have been handled before")
                 assert(stop == nil, "This should have been handled before")
+                // Special case: } terminates implicit table (envName == nil) created by \\
+                // This happens when \\ is used inside braces: \substack{a \\ b}
+                if self.currentEnv != nil && self.currentEnv!.envName == nil {
+                    // Mark environment as ended, don't consume the }
+                    self.currentEnv!.ended = true
+                    return list
+                }
                 // We encountered a closing brace when there is no stop set, that means there was no
                 // corresponding opening brace.
                 self.setError(.mismatchBraces, message:"Mismatched braces.")
@@ -740,6 +747,35 @@ public struct MTMathListBuilder {
             let under = MTUnderLine()
             under.innerList = self.buildInternal(true)
             return under
+        } else if command == "substack" {
+            // \substack reads ONE braced argument containing rows separated by \\
+            // Similar to how \frac reads {numerator}{denominator}
+
+            // Read the braced content using standard pattern
+            let content = self.buildInternal(true)
+
+            if content == nil {
+                return nil
+            }
+
+            // The content may already be a table if \\ was encountered
+            // Check if we got a table from the \\ parsing
+            if content!.atoms.count == 1, let tableAtom = content!.atoms.first as? MTMathTable {
+                return tableAtom
+            }
+
+            // Otherwise, single row - wrap in table
+            var rows = [[MTMathList]]()
+            rows.append([content!])
+
+            var error: NSError? = self.error
+            let table = MTMathAtomFactory.table(withEnvironment: nil, rows: rows, error: &error)
+            if table == nil && self.error == nil {
+                self.error = error
+                return nil
+            }
+
+            return table
         } else if command == "begin" {
             let env = self.readEnvironment()
             if env == nil {

Tests/SwiftMathTests/MTMathListBuilderTests.swift

@@ -2174,22 +2174,53 @@ final class MTMathListBuilderTests: XCTestCase {
 
     func testSubstack() throws {
         // Test \substack for multi-line subscripts and limits
+
         let testCases = [
+            ("\\substack{a \\\\ b}", "simple substack"),
+            ("x_{\\substack{a \\\\ b}}", "substack in subscript"),
             ("\\sum_{\\substack{0 \\le i \\le m \\\\ 0 < j < n}} P(i,j)", "substack in sum limits"),
             ("\\prod_{\\substack{p \\text{ prime} \\\\ p < 100}} p", "substack with text"),
-            ("A_{\\substack{n \\\\ k}}", "substack in subscript")
+            ("A_{\\substack{n \\\\ k}}", "subscript with substack"),
+            ("\\substack{\\frac{a}{b} \\\\ c}", "substack with frac"),
+            ("\\substack{a}", "single row substack"),
+            ("\\substack{a \\\\ b \\\\ c \\\\ d}", "multi-row substack")
         ]
 
         for (latex, desc) in testCases {
+            print("Testing: \(desc)")
+            print("  LaTeX: \(latex)")
             var error: NSError? = nil
             let list = MTMathListBuilder.build(fromString: latex, error: &error)
 
-            if list == nil || error != nil {
-                throw XCTSkip("\\substack not implemented: \(desc). Error: \(error?.localizedDescription ?? "nil result")")
+            if let err = error {
+                print("  ERROR: \(err.localizedDescription)")
+            } else if list == nil {
+                print("  List is nil but no error")
+            } else {
+                print("  SUCCESS: Got \(list!.atoms.count) atoms")
             }
 
             let unwrappedList = try XCTUnwrap(list, "Should parse: \(desc)")
+            XCTAssertNil(error, "Should not error on \(desc): \(error?.localizedDescription ?? "")")
             XCTAssertTrue(unwrappedList.atoms.count >= 1, "\(desc) should have atoms")
+
+            // Verify we have a table structure (either directly or in subscript)
+            var foundTable = false
+            for atom in unwrappedList.atoms {
+                if atom.type == .table {
+                    foundTable = true
+                    break
+                }
+                if let subScript = atom.subScript {
+                    for subAtom in subScript.atoms {
+                        if subAtom.type == .table {
+                            foundTable = true
+                            break
+                        }
+                    }
+                }
+            }
+            XCTAssertTrue(foundTable, "\(desc) should contain a table structure")
         }
     }