adding documentation

Author: mmatera

CHANGES.rst

@@ -50,7 +50,8 @@ Bugs
 
 # ``0`` with a given precision (like in ```0`3```) is now parsed as ``0``, an integer number.
 #. ``RandomSample`` with one list argument now returns a random ordering of the list items. Previously it would return just one item.
-#. Rules of the form ``pat->Condition[expr, cond]`` are handled as in WL. The same works for nested `Condition` expressions.
+#. Rules of the form ``pat->Condition[expr, cond]`` are handled as in WL. The same also works for nested `Condition` expressions. In particular, the comparison between two Rules with the same pattern but an iterated ``Condition`` expressionare considered equal if the conditions are the same. 
+   
 
 Enhancements
 ++++++++++++

mathics/builtin/assignments/assignment.py

@@ -170,11 +170,21 @@ class SetDelayed(Set):
     'Condition' ('/;') can be used with 'SetDelayed' to make an
     assignment that only holds if a condition is satisfied:
     >> f[x_] := p[x] /; x>0
+    >> f[x_] := p[-x]/; x<-2 
     >> f[3]
      = p[3]
     >> f[-3]
-     = f[-3]
-    It also works if the condition is set in the LHS:
+     = p[3]
+    >> f[-1]
+     = f[-1]
+    Notice that the LHS is the same in both definitions, but the second
+    does not overwrite the first one.
+
+    To overwrite one of these definitions, we have to assign using the same condition:
+    >> f[x_] := Sin[x] /; x>0
+    >> f[3]
+     = Sin[3] 
+    In a similar way, the condition can be set in the LHS:
     >> F[x_, y_] /; x < y /; x>0  := x / y;
     >> F[x_, y_] := y / x;
     >> F[2, 3]

mathics/core/definitions.py

@@ -802,10 +802,15 @@ def get_tag_position(pattern, name) -> Optional[str]:
 
 
 def insert_rule(values, rule) -> None:
+    rhs_cond = getattr(rule, "rhs_conditions", [])
     for index, existing in enumerate(values):
         if existing.pattern.sameQ(rule.pattern):
-            del values[index]
-            break
+            # Check for coincidences in the rhs conditions,
+            # it they are there.
+            existing_rhs_cond = getattr(existing, "rhs_conditions", [])
+            if existing_rhs_cond == rhs_cond:
+                del values[index]
+                break
     # use insort_left to guarantee that if equal rules exist, newer rules will
     # get higher precedence by being inserted before them. see DownValues[].
     bisect.insort_left(values, rule)

mathics/core/rules.py

@@ -251,8 +251,9 @@ def __init__(self, pattern, replace, delayed=True, system=False) -> None:
             while replace.has_form("System`Condition", 2):
                 replace, cond = replace.elements
                 conds.append(cond)
-            self.rhs_conditions = sorted(conds)
-            self.strip_replace = replace
+
+        self.rhs_conditions = sorted(conds)
+        self.strip_replace = replace
 
     def do_replace(self, expression, vars, options, evaluation):
         replace = self.replace if self.rhs_conditions == [] else self.strip_replace