Get PyMathics minimally working

Works for both pymathics-hello and pymathics-natlang

However pattern parsing is a little funky and should be improved.

Author: rocky

mathics/builtin/base.py

@@ -33,16 +33,17 @@ def valid_context_name(ctx, allow_initial_backquote=False) -> bool:
             (allow_initial_backquote or not ctx.startswith('`')))
 
 
-def ensure_context(name) -> str:
+def ensure_context(name, context="System`") -> str:
     assert isinstance(name, str)
     assert name != ''
     if '`' in name:
         # Symbol has a context mark -> it came from the parser
         assert fully_qualified_symbol_name(name)
         return name
     # Symbol came from Python code doing something like
-    # Expression('Plus', ...) -> use System`
-    return 'System`' + name
+    # Expression('Plus', ...) -> use System` or more generally
+    # context + name
+    return context + name
 
 
 def strip_context(name) -> str:

mathics/core/definitions.py

@@ -44,9 +44,19 @@ def lookup_name(self, name):
         return ensure_context(name)
 
 
-def parse_builtin_rule(string):
+class PyMathicsDefinitions(object):
+    """
+    Dummy Definitions object that puts every unqualified symbol in
+    PyMathics`.
+    """
+    def lookup_name(self, name):
+        assert isinstance(name, str)
+        return ensure_context(name, "PyMathics`")
+
+
+def parse_builtin_rule(string, definitions=SystemDefinitions()):
     '''
     Parse rules specified in builtin docstrings/attributes. Every symbol
     in the input is created in the System` context.
     '''
-    return parse(SystemDefinitions(), SingleLineFeeder(string, '<builtin_rules>'))
+    return parse(definitions, SingleLineFeeder(string, '<builtin_rules>'))