Fix Factor[a == b] (#232)

* Fix Factor[a == b]
* Improve Factor documentation

Author: TiagoCavalcante

mathics/builtin/numbers/algebra.py

@@ -576,9 +576,21 @@ class Factor(Builtin):
     >> Factor[1 / (x^2+2x+1) + 1 / (x^4+2x^2+1)]
      = (2 + 2 x + 3 x ^ 2 + x ^ 4) / ((1 + x) ^ 2 (1 + x ^ 2) ^ 2)
 
-    ## Issue659
-    #> Factor[{x+x^2}]
-     = {x (1 + x)}
+    Factor can also be used with equations:
+    >> Factor[x a == x b + x c]
+     = a x == x (b + c)
+
+    And lists:
+    >> Factor[{x + x^2, 2 x + 2 y + 2}]
+     = {x (1 + x), 2 (1 + x + y)}
+
+    It also works with more complex expressions:
+    >> Factor[x ^ 3 + 3 x ^ 2 y + 3 x y ^ 2 + y ^ 3]
+     = (x + y) ^ 3
+
+    You can use Factor to find when a polynomial is zero:
+    >> x^2 - x == 0 // Factor
+     = x (-1 + x) == 0
     """
 
     attributes = listable | protected
@@ -592,14 +604,9 @@ def apply(self, expr, evaluation):
 
         try:
             result = sympy.together(expr_sympy)
-            numer, denom = result.as_numer_denom()
-            if denom == 1:
-                result = sympy.factor(expr_sympy)
-            else:
-                result = sympy.factor(numer) / sympy.factor(denom)
+            return from_sympy(sympy.factor(result))
         except sympy.PolynomialError:
             return expr
-        return from_sympy(result)
 
 
 class FactorTermsList(Builtin):