Start CheckAbort[] builtin function (#1111)

Just basic implementation. Right now, we don't handle options.

CHANGES.rst

@@ -6,6 +6,7 @@ CHANGES
 New Builtins
 ++++++++++++
 
+* ``CheckAbort``
 * ``SetEnvironment``
 
 Compatibility
@@ -22,9 +23,9 @@ Operator precedence has been gone over and is picked up in tables from the Mathi
 Performance
 -----------
 
-* `Blank*` patterns without arguments are now singletons.
+* ``Blank*`` patterns without arguments are now singletons.
+
 
-
 7.0.0
 -----
 

mathics/builtin/procedural.py

@@ -152,6 +152,36 @@ def eval_with_form_and_fn(self, expr, form, f, evaluation):
         return ret
 
 
+class CheckAbort(Builtin):
+    """
+    <url>:WMA link:
+    https://reference.wolfram.com/language/ref/CheckAbort.html</url>
+
+    <dl>
+      <dt>'CheckAbort[$expr$, $failexpr$]'
+        <dd>evaluates $expr$, returning $failexpr$ if an abort occurs.
+    </dl>
+
+    >> CheckAbort[Abort[]; 1, 2] + x
+     = 2 + x
+
+    >> CheckAbort[1, 2] + x
+     = 1 + x
+    """
+
+    attributes = A_HOLD_ALL | A_PROTECTED
+
+    summary_text = "catch an Abort[] exception"
+
+    def eval(self, expr, failexpr, evaluation):
+        "CheckAbort[expr_, failexpr_]"
+
+        try:
+            return expr.evaluate(evaluation)
+        except AbortInterrupt:
+            return failexpr
+
+
 class CompoundExpression(BinaryOperator):
     """
     <url>:WMA link: