Docfixes part2 (#1300)

Doctest corrections for Arg, FilenameDrop and SetEnvironment. 

SetEnvironment return value corrected.

mathics/builtin/arithmetic.py

@@ -122,15 +122,15 @@ class Arg(MPMathFunction):
      >> Arg[-3]
       = Pi
 
-     Same as above using sympy's method:
+     Same as above, but using SymPy's method:
      >> Arg[-3, Method->"sympy"]
       = Pi
 
     >> Arg[1-I]
      = -Pi / 4
 
-    Arg evaluate the direction of DirectedInfinity quantities by
-    the Arg of they arguments:
+    'Arg' evaluates the direction of 'DirectedInfinity' quantities by \
+    the 'Arg' of its arguments:
     >> Arg[DirectedInfinity[1+I]]
      = Pi / 4
     >> Arg[DirectedInfinity[]]

mathics/builtin/file_operations/path_operations.py

@@ -34,7 +34,7 @@ class FileNameDrop(Builtin):
       <dd>drops the last path elements in the file name $path$.
     </dl>
 
-    >> path = FileNameJoin{"a","b","c"}
+    >> path = FileNameJoin[{"a","b","c"}]
      = ...
 
     >> FileNameDrop[path, -1]

mathics/builtin/system.py

@@ -20,6 +20,7 @@
 from mathics.core.evaluation import Evaluation
 from mathics.core.expression import Expression
 from mathics.core.list import ListExpression
+from mathics.core.symbols import SymbolNull
 from mathics.core.systemsymbols import (
     SymbolFailed,
     SymbolNone,
@@ -569,7 +570,6 @@ class SetEnvironment(Builtin):
 
      Set a single environment variable:
      S> SetEnvironment["FOO" -> "bar"]
-      = SetEnvironment[FOO -> bar]
 
      See that the environment variable has changed:
      S> GetEnvironment["FOO"]
@@ -587,18 +587,14 @@ class SetEnvironment(Builtin):
 
      S> SetEnvironment["FOO" -> 5]
       : 5 must be a string or None.
-      = SetEnvironment[FOO -> 5]
+      = $Failed
 
      S> GetEnvironment["FOO"]
       = FOO -> baz
 
     If the environment name is not a string, the evaluation fails without a message.
 
-     S> SetEnvironment[FOO -> "bar"]
-      = SetEnvironment[FOO -> bar]
-
-     S> GetEnvironment["FOO"]
-      = FOO -> baz
+     S> SetEnvironment[1 -> "bar"]
 
      See also <url>
      :'Environment':
@@ -615,14 +611,14 @@ def eval(self, rule, evaluation):
         env_var_name, env_var_value = rule.elements
         if not (env_var_value is SymbolNone or isinstance(env_var_value, String)):
             evaluation.message("SetEnvironment", "value", env_var_value)
-            return None
+            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
             )
-        return None
+        return SymbolNull
 
     def eval_list(self, rules: Expression, evaluation: Evaluation):
         "SetEnvironment[{rules__}]"