So currently the p_arith_command and _extractcommandsubst functions pop a NotImplemented errors when an arithmetic expression is found. In bash-master/make_cmd line 430, the make_arith_command function is implemented simply to set the .value attribute equal to the string, the flags to zero, give the node type cm_arith, and set the redirects to null. Would adding this implementation into the p_arith_command function be an acceptable fix? subst.py would also need to be changed to implement these functions. If you just call _parsedelparen on the airthmetic expression, the parsing seems to work just fine. The parens shouldn't be parsed as nodes and the node type should be 'arith_cmd' but those are easy fixes. Is there something I am missing as to why these aren't implemented?