[fix] invalid link variables

Author: tswsxk

tests/test_formula.py

@@ -1,6 +1,6 @@
 # coding: utf-8
 # 2021/5/20 @ tongshiwei
-
+import pytest
 from EduNLP.Formula import Formula, FormulaGroup
 
 
@@ -9,15 +9,26 @@ def test_formula():
     f = Formula(formula)
     f.variable_standardization(inplace=False)
     f.variable_standardization(inplace=True)
-    assert len(f.ast.nodes) == len(f.element)
+    assert len(f.ast_graph.nodes) == len(f.ast)
     f.to_str()
 
     formula = r"\frac{\pi}{2}"
     f = Formula(formula, variable_standardization=True)
     assert repr(f) == r"<Formula: \frac{\pi}{2}>"
 
+    f = Formula(f.ast)
+    assert f.resetable is False
+    with pytest.raises(TypeError):
+        f.reset_ast()
+
     fg = FormulaGroup([r"x + x", r"x + \frac{\pi}{2}"], variable_standardization=True)
     for f in fg:
         assert f in fg
-    assert len(fg[0].element) == 3
+    assert len(fg[0].ast) == 3
     fg.to_str()
+
+    fg = FormulaGroup(["x", "y", "x"])
+    assert len(fg.ast) == 3
+
+    with pytest.raises(TypeError):
+        FormulaGroup([{}])