Added validation functions.

src/validation.py

+from src.parser import Parser
+from tests.parser import ParserWrapper
+
+
+class ValidationNode(object):
+    pass
+
+
+def validate(exp, result):
+    """
+    Validate that exp =>* result.
+    """
+    parser = ParserWrapper(Parser)
+
+    exp = parser.run([exp])
+    result = parser.run([result])
+
+    return validate_graph(exp, result)
+
+
+def iter_preorder(exp, possibility):
+    """
+    Traverse the possibility tree using pre-order traversal.
+    """
+    pass
+
+
+def validate_graph(exp, result):
+    """
+    Validate that "exp" =>* "result".
+    """
+    # TODO: Traverse the tree of possibility applications
+    return False