Test: add doctest for a bug about tree encapsulation

mapytex/calculus/API/expression.py

@@ -99,6 +99,10 @@ class Expression(object):
         <Exp: -3 / -15>
         >>> Expression.random("{a}/{a*k} - 3*{b}", variables_scope={'a':{'min_max':(10, 30)}}) # doctest: +SKIP
         <Exp: 18 / 108 - 3 * 9>
+        >>> e = Expression.random("{a}*x + {b}*x + 3")
+        >>> ee = e.simplify()
+        >>> print(e) # doctest: +SKIP
+        >>> print(ee) # doctest: +SKIP
 
         """
         rd_t = Tree.from_str(template, random=True)