adjust SetEnvironment (#1429)

This is a first example of implementation that works by chance due to a
general misimplementation.
Also, now the result is compliant with WMA.

---------

Co-authored-by: R. Bernstein <rocky@users.noreply.github.com>

Author: mmatera

mathics/builtin/system.py

@@ -603,7 +603,6 @@ class SetEnvironment(Builtin):
 
      Set two environment variables:
      S> SetEnvironment[{"FOO" -> "baz", "A" -> "B"}]
-      = SetEnvironment[{FOO -> baz, A -> B}]
 
      See that the environment variable has changed:
      S> GetEnvironment["FOO"]
@@ -621,6 +620,7 @@ class SetEnvironment(Builtin):
     If the environment name is not a string, the evaluation fails without a message.
 
      S> SetEnvironment[1 -> "bar"]
+      = SetEnvironment[1 -> bar]
 
      See also <url>
      :'Environment':
@@ -633,24 +633,35 @@ class SetEnvironment(Builtin):
     summary_text = "set system environment variable(s)"
 
     def eval(self, rule, evaluation):
-        "SetEnvironment[rule_]"
+        "SetEnvironment[rule_Rule]"
         env_var_name, env_var_value = rule.elements
+        # WMA does not give an error message if env_var_name is not a String - weird.
+        if not isinstance(env_var_name, String):
+            return None
+
         if not (env_var_value is SymbolNone or isinstance(env_var_value, String)):
             evaluation.message("SetEnvironment", "value", env_var_value)
             return SymbolFailed
 
-        if isinstance(env_var_name, String):
-            # WMA does not give an error message if env_var_name is not a String - weird.
-            os.environ[env_var_name.value] = (
-                None if None is SymbolNone else env_var_value.value
-            )
+        os.environ[env_var_name.value] = (
+            None if None is SymbolNone else env_var_value.value
+        )
         return SymbolNull
 
     def eval_list(self, rules: Expression, evaluation: Evaluation):
         "SetEnvironment[{rules__}]"
+
+        # All the rules must be of the form
+        for rule in rules.elements:
+            if not rule.has_form("System`Rule", 2):
+                return None
+            if not isinstance(rule.elements[0], String):
+                return None
+
         for rule in rules.elements:
             self.eval(rule, evaluation)
-        return None
+
+        return SymbolNull
 
 
 class Share(Builtin):