Add test for random_expression and underscore

pymath/calculus/test/test_random_expression.py

@@ -17,6 +17,17 @@ def test_only_form():
     assert set(rdExp._gene_varia.keys()) == {'a'} 
     assert set(rdExp._gene_2replaced.keys()) == {'a'} 
 
+def test_form_with_underscores():
+    form = "_ + 2*_"
+    rdExp = RdExpression(form)
+
+    assert rdExp._letters == {'A', 'B'}
+    assert rdExp._2replaced == {'A', 'B'}
+
+    rdExp()
+    assert set(rdExp._gene_varia.keys()) == {'A', 'B'} 
+    assert set(rdExp._gene_2replaced.keys()) == {'A', 'B'} 
+
 def test_only_form_calc():
     form = "{a+b} + 2"
     rdExp = RdExpression(form)