disable regressions in ST mode

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

azure-pipelines.yml

@@ -26,8 +26,10 @@ jobs:
     matrix:
       MT:
         cmdLine: 'python scripts/mk_make.py -d --java --dotnet'
+        runRegressions: 'True'
       ST:
         cmdLine: './configure --single-threaded'
+        runRegressions: 'False'
   steps:
     - script: $(cmdLine)
     - script: |
@@ -38,7 +40,8 @@ jobs:
         make -j3 test-z3
         cd ..
     - template: scripts/test-z3.yml
-    - template: scripts/test-regressions.yml
+    - ${{if eq(variables['runRegressions'], 'True')}}:
+      - template: scripts/test-regressions.yml
     - template: scripts/generate-doc.yml
 
 - job: "Ubuntu18Python"

scripts/test-examples-cmake.yml

@@ -9,7 +9,7 @@ steps:
     examples/c_example_build_dir/c_example
     examples/cpp_example_build_dir/cpp_example
     examples/tptp_build_dir/z3_tptp5 -help
+    echo examples/c_maxsat_example_build_dir/c_maxsat_example ../examples/maxsat/ex.smt
     cd ..
 
 
-#   examples/c_maxsat_example_build_dir/c_maxsat_example ../examples/maxsat/ex.smt

src/ast/rewriter/seq_rewriter.cpp

@@ -2010,8 +2010,8 @@ expr_ref seq_rewriter::is_nullable(expr* r) {
         result = mk_or(m(), is_nullable(r1), is_nullable(r2));
     }
     else if (m_util.re.is_diff(r, r1, r2)) {
-        expr_ref e2(mk_not(m(), is_nullable(r2)), m());
-        result = mk_and(m(), is_nullable(r1), e2);
+        result = mk_not(m(), is_nullable(r2));
+        result = mk_and(m(), is_nullable(r1), result);
     }
     else if (m_util.re.is_star(r) || 
         m_util.re.is_opt(r) ||