Fix(Render): Ajust "*" visibility with variables

mapytex/calculus/API/__init__.py

@@ -20,6 +20,7 @@ Generate and compute like a student!
 2 + 3 * 4
 2 + 12
 14
+
 >>> e = Expression.from_str("2+3/2")
 >>> e_simplified = e.simplify()
 >>> print(e_simplified)
@@ -32,6 +33,7 @@ Generate and compute like a student!
 4 / 2 + 3 / 2
 (4 + 3) / 2
 7 / 2
+
 >>> e = Expression.from_str("(2/3)^4")
 >>> e_simplified = e.simplify()
 >>> print(e_simplified)
@@ -41,6 +43,7 @@ Generate and compute like a student!
 (2 / 3)^4
 2^4 / 3^4
 16 / 81
+
 >>> e = Expression.from_str("x^2*x*x^4")
 >>> e_simplified = e.simplify()
 >>> print(e_simplified)