robustify `pair_node_to_var` for formulae that include arithmetic
robustify `pair_node_to_var` for formulae that include arithmetic

necozay/tulip-control,tulip-control/tulip-control,tulip-control/tulip-control,necozay/tulip-control,necozay/tulip-control,tulip-control/tulip-control,tulip-control/tulip-control,necozay/tulip-control,necozay/tulip-control