Function par_arith_wordcode 

pub fn par_arith_wordcode()

Port of the case DINPAR: arm of par_cmd from Src/parse.c:1031-1034:

ecadd(WCB_ARITH());
ecstr(tokstr);
zshlex();

(( EXPR )) arithmetic at command position — emits the ARITH opcode followed by the interned EXPR string, then advances past the DINPAR token (which already carries the body text).