Add arg checking to Curl... (#1472)

We use Curl as a developer doc example, so we should have this more correct.

Author: rocky

mathics/builtin/vectors/math_ops.py

@@ -115,6 +115,11 @@ class Curl(SympyFunction):
     """
 
     attributes = A_PROTECTED
+
+    # Set checking that the number of arguments required is two or three.
+    eval_error = Builtin.generic_argument_error
+    expected_args = (2, 3)
+
     rules = {
         "Curl[{f1_, f2_}, {x1_, x2_}]": " D[f2, x1] - D[f1, x2]",
         "Curl[{f1_, f2_, f3_}, {x1_, x2_, x3_}]": """{

test/builtin/vectors/__init__.py

@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-

test/builtin/vectors/test_mathops.py

@@ -2,11 +2,13 @@
 """
 Unit tests for mathics.builtin.vectors.math_ops
 
-In particular, Norm[].
+In particular, Curl[] and Norm[].
 """
 
 from test.helper import check_evaluation
 
+import pytest
+
 
 def test_norm():
     for str_expr, expected_message in (
@@ -48,3 +50,31 @@ def test_norm():
         ),
     ):
         check_evaluation(str_expr=str_expr, str_expected=str_expected)
+
+
+@pytest.mark.parametrize(
+    ("str_expr", "msgs", "assert_fail_msg"),
+    [
+        (
+            "Curl[a]",
+            ["Curl called with 1 argument; 2 or 3 arguments are expected."],
+            "Curl with one argument",
+        ),
+        (
+            "Curl[a, b, c, d]",
+            ["Curl called with 4 arguments; 2 or 3 arguments are expected."],
+            "Curl with more than three arguments",
+        ),
+    ],
+)
+def test_wrong_number_of_arguments(str_expr, msgs, assert_fail_msg):
+    """ """
+    check_evaluation(
+        str_expr,
+        str_expr,
+        to_string_expr=True,
+        to_string_expected=True,
+        hold_expected=True,
+        failure_message=assert_fail_msg,
+        expected_messages=msgs,
+    )