--- interpreterbase.py.orig	2019-08-13 17:41:05.497165796 +0300
+++ interpreterbase.py	2019-08-13 17:41:29.737166739 +0300
@@ -1061,7 +1061,7 @@
             raise InvalidCode('First argument to set_variable must be a string.')
         if not self.is_assignable(variable):
             raise InvalidCode('Assigned value not of assignable type.')
-        if re.match('[_a-zA-Z][_0-9a-zA-Z]*$', varname) is None:
+        if re.match('[+_a-zA-Z][+_0-9a-zA-Z]*$', varname) is None:
             raise InvalidCode('Invalid variable name: ' + varname)
         if varname in self.builtin:
             raise InvalidCode('Tried to overwrite internal variable "%s"' % varname)